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= Turbulent flow past a smooth and rigid wall-mounted hemisphere =
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==  Semi-confined flows ==
 
=== Underlying Flow Regime 3-33 ===
= Description =
= Description =
<!--{{LoremIpsum}}-->
<!--{{LoremIpsum}}-->
== Introduction ==
== Introduction ==
{{Demo_UFR_Desc_Intro}}
 
In environmental and civil engineering, surface-mounted hemispherical bluff bodies are commonly used as architectural design elements. They appear in various applications such as depicted in Fig. 2. Although the hemisphere represents a simple geometry, it exhibits a rather complicated flow field including complex flow patterns. They can be roughly classified into an upstream horseshoe vortex system and a recirculation area with trailing vortices in the wake region. Furthermore, full-scale structures are often exposed to turbulent boundary layers, which increases the complexity of the flow field. The specific flow problem of a wall-mounted hemisphere was studied since the 1940s. However, due to its complexity, most of the contributions focus on a certain part of the flow. The present study published in Wood et al. (2016) includes experimental investigations and large-eddy simulations (LES) to characterize the whole three-dimensional flow field around a surface-mounted smooth hemisphere in a turbulent boundary layer at Re = 50,000.
 
[[Image:UFR3-33_dome_examples.png|x300px]]
 
Fig. 2: Examples of hemispherical domes in modern civil engineering.
 
== Review of previous work ==
== Review of previous work ==


The following brief literature review is extracted from Wood et al. (2016). A full version
can be found in the paper.


Beginning with an analysis based on the pressure distribution the
=== Previous experimental investigations ===
first traceable experiment was carried out by Jacobs in 1938. The measurements focused on the effects of surface roughness
caused by a small hemispherical rivet. Later on Maher carried out
investigations on a series of hemispheres that were placed on the
ground of a wind tunnel in a boundary layer. After exceeding a
Reynolds number (All mentioned Reynolds numbers are based on the diameter of the hemisphere) of <math>Re = 1.6 \times 10^{6}</math> the surface drag coefficient showed no further variations due to
supercritical flow conditions. Similar results were observed in a
comprehensive study by Taylor (1992) confirming the effect
of the Reynolds number independency after exceeding <math>Re = 2 \times 10^{5}</math> and additionally surpassing a turbulence intensity of
4%. Another experiment by Taniguchi et al. (1982)
found that there is a relationship between the approaching boundary layer thickness and the
aerodynamic forces acting on the hemisphere. Furthermore, Cheng et
al. (2010) conducted intensive surface pressure measurements
in a specialized boundary layer wind tunnel. The turbulent boundary
layer featured suburban flow field characteristics with large
turbulence intensities ranging between 18% and 25%. The Reynolds
numbers for the turbulent conditions varied between <math>Re = 5.3 \times 10^{4}</math> and <math>1.7 \times 10^{6}</math> depending on the
size of the hemisphere. The outcome encourages the previous
observations made by Maher (1965) and Taylor (1992).


Besides focusing on pressure measurements Toy et al. (1983)
Beginning with an analysis based on the pressure distribution the first experiments were carried out by Jacobs (1938), Maher (1965) and later on by Taylor (1992) to mention only a few. They showed Reynolds number independency above a critical Re number and turbulence intensity. Another experiment by Taniguchi et al. (1982) found a relationship between the approaching boundary layer thickness and the aerodynamic forces acting on the hemisphere. Toy et al. (1983) and Savory and Toy (1986) investigated the flow past a hemispherical dome using hot-wire and pulsed-wire anemometers and brought a deeper insight into the complex flow structures occurring in the near-wake regime. Another experiment by Savory and Toy (1988) focused on the separation of the shear layer in the flow around hemispheres at a sub-critical Reynolds number of Re = 140,000. The study included different turbulent boundary layers classified in thin, smooth and rough boundary layers depending on the thickness and the turbulence intensity. A profound discussion provided a deeper understanding of the distribution of the turbulent shear stress and intensity in the wake regime. Additionally, a representative illustration of the flow field characteristics was provided. The outcome of both studies (Savory and Toy 1986, 1988) is often used as reference for experimental and numerical examinations.
initially investigated the flow field past a hemispherical dome using
hot-wire and pulsed-wire anemometers. Based on this investigation
Savory and Toy (1986) brought a yet deeper insight into the
complex flow structures occurring in the near-wake regime of
hemispheres and cylinders with hemispherical caps that were exposed to
three different turbulent boundary layers within the range from
\mbox{Re = 4.31 $\times$ 10$^{4}$} to \mbox{1.4 $\times$
  10$^{5}$}. The investigation included the effects of surface
roughness of the model on the drag coefficient as well as the velocity
field of the recirculation area past the hemisphere. A second
experiment conducted by Savory and Toy~\cite{savory1988} focused on
the separation of the shear layer in the flow around hemispheres
conducted at a sub-critical Reynolds number of \mbox{Re = 1.4 $\times$
  10$^{5}$}. The study included different turbulent boundary layers
classified in thin, smooth and rough boundary layers depending on the
thickness and the turbulence intensity. To generate the desired inflow
characteristics, Savory and Toy applied artificial boundary layer
installations including fences and vortex generators in order to
investigate the influence of the upstream boundary conditions. A
profound discussion provided a deeper understanding of the
distribution of the turbulent shear stress and intensity in the wake
regime. Additionally, a representative illustration of the flow field
characteristics was made that includes the horseshoe vortex system,
the trailing vortices and the separation regions. The outcome of both
studies~\cite{savory1986,savory1988} is often used as reference for
experimental and numerical examinations. A comparable schematic view
is provided by Martinuzzi and Tropea~\cite{martinuzzi1993} in case of
the flow past a wall-mounted cube, as well as by Pattenden et
al.~\cite{pattenden2005} for the flow around a wall-mounted
cylinder. Indeed, both cases possess similar separation and
reattachment characteristics.


%%%%%%%%%%%%%%%%%%%%%%% Experiments on Visualization of Flow Patterns %%%%%%%%%%%%
Visualizations of the vortical flow structures were conducted by Tamai et al. (1987). The experiments allowed to visualize the complex vortical structures by injecting dye into the water channel. Moreover, the frequencies of the vortex formation and shedding from the separation area were recorded. Bennington (2004) examined various roughness elements and their associated effects on the turbulent boundary layer. Among the chosen elements, a hemispherical obstacle was analyzed in detail concerning statistics of the Reynolds stresses, the turbulent kinetic energy and even the triple correlations. Further visualization experiments were conducted by Yaghoubi (1991). They comprised a detailed visualization of the flow pattern around grouped hemispheres in a wind tunnel. The motivation was to achieve a deeper understanding of the flow field and the associated effects of natural ventilation of domed structures.
Primary visualizations of the vortical flow structures were conducted
by Tamai et al.~\cite{tamai1987}. The experimental setup included two
hemispheres of different size exposed to water flow in the range
\mbox{2 $\times$ 10$^{2}$ $<$ Re $<$ 1.2 $\times$ 10$^{4}$}. The
experiments allowed to visualize the complex vortical structures by
injecting dye into the water channel. Moreover, the frequencies of the
vortex formation and shedding from the separation area were recorded
by measuring the spectra of the velocity fluctuations inside and
outside the recirculation zone. Another observation was made by
Acarlar and Smith~\cite{acarlar1987} who carried out elaborate
experiments using relatively small hemispheres in the laminar flow
regime to generate hairpin vortices. It turned out that the downstream
velocity profiles resulting from the artificially induced flow
structures were similar to those of a turbulent boundary
layer. Bennington~\cite{bennington2004} examined various roughness
elements and their associated effects on the turbulent boundary
layer. Among the chosen elements, a hemispherical obstacle is
analyzed in detail concerning statistics of the Reynolds stresses,
the turbulent kinetic energy and even the triple correlations.


Simpson et al.~\cite{simpson2002} examined the flow separation at an
=== Previous numerical investigations ===
axisymmetric bump by utilizing surface mean pressure measurements, oil
flow visualizations and laser-Doppler measurements. The results showed
a nearly symmetric mean flow over the bump including a detailed
mapping of separation and nodal points on the leeside of the
obstacle. Furthermore, Byun and Simpson~\cite{byun2006} intensified
the research on the bump to characterize the 3D separations by using a
fine-spatial-resolution laser-Doppler system and later \cite{byun2010}
supplemented the studies by adding an investigation on the pressure
fluctuations. Similar to the flow past the hemisphere a
pressure-driven separation occurs. However, the separation line is
shifted further downstream and reattachment is much earlier, leading
to a smaller recirculation area.


Further visualization experiments were conducted by
Apart from experimental investigations numerical simulations were carried out to provide enhanced insight into the flow. An early study was conducted by Tamura et al. (1990) without applying any turbulence model. The focus of the simulations lay on the visualization of the unsteady flow pattern and the time-averaged surface pressure distribution. A fundamental numerical study was carried out by Manhart (1998) using large-eddy simulation to receive more detailed information about the vortical structures. The Cartesian grid combined with the immersed boundary technique led to an artificial surface roughness on the contour of the hemisphere. The results were therefore compared with the experiments of Savory and Toy (1986, 1988) for a rough hemisphere. Another comparison of numerical and experimental data was made by Meroney et al. (2002). The wind load distributions on smooth, rough and dual domes in the shape of hemispherical caps were examined. Several RANS turbulence models were used delivering similar results. Recently, Kharoua and Khezzar (2013) performed a LES on a hemisphere with a rough and smooth surface comparing the results with the experiment of Savory and Toy (1986). A specialized approach to model the surface roughness was presented. The results of the LES allowed the visualization of instantaneous three-dimensional flow patterns illustrating the complex interaction of vortical structures in the close vicinity of the hemisphere.
Yaghoubi~\cite{yaghoubi1991}. They comprised a detailed visualization
of the air flow pattern around grouped hemispheres in a wind
tunnel. The motivation for the study was to achieve a deeper
understanding of the flow field and the associated effects of natural
ventilation of domed structures often appearing in oriental
architecture.


%%%%%%%%%%%%%%%%%% Numerical Investigations %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
=== Previous complementary experimental and numerical investigations ===
Apart from experimental investigations numerical simulations were
carried out to provide enhanced insight into the flow. An early study
was conducted by Tamura et al.~\cite{tamura1990} without applying any
turbulence model. The focus of the simulations lay on the
visualization of the unsteady flow pattern and the time-averaged
surface pressure distribution at \mbox{Re = 2 $\times$ 10$^3$} and
\mbox{2 $\times$ 10$^4$}, respectively.


A fundamental numerical study was carried out by
A combined experimental and numerical study was accomplished by Tavakol et al. (2010). A hemisphere was immersed in two turbulent boundary layers of different thickness. Velocity distributions and turbulence intensities were presented for the streamwise and the spanwise directions in the recirculation zone. A further velocity measurement was carried out for the area close to the front of the hemisphere investigating the horseshoe vortex that leads to a strong backflow in the near-wall region. The numerical investigation relied on the RNG k-&epsilon; turbulence model (Yakhot et al., 1992). The inflow conditions of the simulation were generated by implying the time-averaged data of the corresponding hot-wire measurements. The turbulence intensity at the inlet was also taken from the measurements. The results showed overall good agreement with the experimental data. Recently, Tavakol et al. (2014) presented a yet deeper investigation of the hemisphere flow using LES. Based on the earlier study (Tavakol et al., 2010) the main focus was to highlight the superior results of the applied LES compared to the previously performed RANS simulations. The study included a thin turbulent boundary layer &delta;/<var>D</var> = 0.15 as inflow condition. For a realistic inlet velocity distribution including fluctuations, a turbulence inflow generator based on the method of Sergent (2002) was applied. The LES results showed excellent agreement with the measurements. An updated comparison between the previous study (Tavakol et al., 2015) with the current data revealed the shortcomings of the RNG k-&epsilon; model. A presentation of time-averaged data focuses on the streamline visualization and surface pressure distribution. Unfortunately, the study does not present statistical data of the velocity field or the Reynolds stresses.
Manhart~\cite{manhart1998} using large-eddy simulation to receive more
detailed information about the vortical structures at \mbox{Re = 1.5
  $\times$ 10$^{5}$}. The Cartesian grid combined with the immersed
boundary technique led to an artificial surface roughness on the
contour of the hemisphere. The results were therefore compared with
the experiments of Savory and Toy~\cite{savory1986, savory1988} for a
rough hemisphere. Besides observations of temporal spectra and the
velocity distributions, the proper orthogonal decomposition method was
applied to examine the highly complex separation processes and to
determine the dominant vortical structures.


Another comparison of numerical and experimental data was made by
== Choice of test case ==
Meroney et al.~\cite{meroney2002}. The three-dimensional wind load
distributions on smooth, rough and dual domes in the shape of
hemispherical caps were examined. The calculations were carried out
for \mbox{Re = 1.85 $\times$ 10$^{5}$} and \mbox{1.44 $\times$
  10$^{6}$}. Several RANS turbulence models including the classical
k-$\epsilon$ model, a Reynolds stress model~\cite{wilcox1998} and the
Spalart-Allmaras model~\cite{spalart1992} were used delivering similar
results. Recently, Kharoua and Khezzar~\cite{kharoua2013} performed a
LES on a hemisphere with a rough and smooth surface at \mbox{Re = 1.4
  $\times$ 10$^{5}$} comparing the results with the experiment of
Savory and Toy~\cite{savory1986}. A specialized approach to model the
surface roughness was presented. The results of the LES allowed the
visualization of instantaneous three-dimensional flow pattern
illustrating the complex interaction of vortical structures in the
close vicinity of the hemisphere. It turned out that the model
roughness leads to a larger recirculation area compared to the smooth
surface.


Garc\'ia-Villalba et al.~\cite{garcia2009} conducted LES to study the
The literature review presented indicates that a surface-mounted hemisphere placed in a turbulent boundary layer exhibits a very complex flow field. The key aspect of most studies listed above often remains on one specific issue such as the recirculation area or the pressure distribution. Just a few studies contain general characteristics of the flow including complementary numerical and experimental investigations.
behavior of turbulent flow separation from an axisymmetric
three-dimensional bump at a Reynolds number of \mbox{Re = 1.3 $\times$
  10$^{5}$}. The characteristics of the turbulent flow field were
compared with the experimental results mentioned
above~\cite{simpson2002,byun2006,byun2010} strongly focusing on the
formation of the separation region on the rear side of the bump.


%%%%%%%%%%%%%%% Numerical and Experimental Investigations %%%%%%%%%%%%%%%%%%%%%%%%
A combined experimental and numerical study was accomplished by
Tavakol et al.~\cite{tavakol2010}. A hemisphere was immersed in two
turbulent boundary layers of different thickness. The experiments were
conducted in a wind tunnel using a hot-wire sensor to record the
velocity field at certain planes upstream and downstream of the
hemisphere at \mbox{Re = 6.4 $\times$ 10$^{4}$}. Velocity
distributions and turbulence intensities were presented for the
streamwise and the spanwise directions in the recirculation zone. A
further velocity measurement was carried out for the area close to the
front of the hemisphere investigating the horseshoe vortex that leads
to a strong backflow in the near-wall region. The numerical
investigation relied on the RNG k-$\epsilon$ turbulence
model~\cite{yakhot1992}. The inflow conditions of the simulation were
generated by implying the time-averaged data of the corresponding
hot-wire measurements. The turbulence intensity at the inlet is also
taken from the measurements. The results showed overall good agreement
with the experimental data. Recently, Tavakol et
al.~\cite{tavakol2014} presented a yet deeper investigation of the
hemisphere flow using LES at \mbox{Re = 3.6 $\times$ 10$^{4}$} and
\mbox{6.4 $\times$ 10$^{4}$}. Based on the earlier
study~\cite{tavakol2010} the main focus of this paper was to highlight
the superior results of the applied LES compared to the previously
performed RANS simulations. The numerical grid consisted of \mbox{4.2
  $\times$ 10$^{6}$} CVs. A detailed analysis of different
subgrid-scale (SGS) models, i.e., WALE~\cite{nicoud1999wale}, dynamic
Smagorinsky~\cite{germano} and the kinetic energy transport
model~\cite{kim1997} was performed. The study included a thin
turbulent boundary layer $\delta/D \approx 0.15$ as inflow
condition. For a realistic inlet velocity distribution including
fluctuations a turbulence inflow generator based on the method of
Sergent~\cite{sergent2002vers} was applied. As a result the LES showed
excellent agreement with the measurements. An updated comparison
between the previous study~\cite{tavakol2014} with the current data
revealed the shortcomings of the RNG k-$\epsilon$ model especially in
the wake of the hemisphere. A presentation of time-averaged data
focuses on the streamline visualization and surface pressure
distribution. Unfortunately, the paper does not present statistical
data of the velocity field or the Reynolds stresses.


== Choice of test case ==
The present UFR is based on the '''experimental LDA and numerical LES study of Wood et al. (2016)''' which focuses on the following objectives:
 
* It provides a comprehensive view of the flow field past a hemispherical object immersed in a turbulent boundary layer at Re = 50,000 with the help of experimental and numerical investigations: All relevant regions of the flow field (horseshoe vortex system, recirculation area and wake) are studied in detail including unsteady characteristics such as vortex shedding and related spectral analysis.
 
* To avoid uncertainties in the numerical model, the surface of the hemisphere is assumed to be smooth. The experimental data are recorded based on a very smooth aluminum model with low engineering tolerances which are expected to minimize possible influences of surface roughness during the measurements.
 
* The characteristics of the oncoming turbulent boundary layer are taken from the wind tunnel measurements and transferred to the numerical domain. This matching ensures the comparability between the experiment and the numerical simulation, focusing on the mean velocity profile and the turbulent fluctuations of the boundary layer. The numerical simulations use a synthetic turbulent inflow generator (STIG) approach to mimic the turbulent fluctuations of the boundary layer. That is found to be an important issue, since the turbulent fluctuations have a significant impact on the overall flow-field charateristics such as reattachment and separation points.
 
* The time-averaged flow field is analyzed in detail, focussing on the mean velocity distribution in the symmetry plane. Furthermore, the corresponding Reynolds stresses of the flow field past the hemisphere are analyzed in detail including a comparison between the experimental measurements and the numerical simulation data.
 
* The effect of well-known and often applied subgid-scale (SGS) models is highlighted to present their influence on the results.
 
* The test case with complementary experimental and numerical data for the surface-mounted hemisphere flow offers a novel benchmark for the evaluation and validation of numerical schemes or new turbulence models.


* Finally, the investigation of the flow past the rigid structure is the first step towards an investigation of the coupled fluid-structure interaction of the flow around a flexible membranous structure. Such as study is intended for the near future. 
<br/>
<br/>
----
----

Latest revision as of 13:48, 12 February 2017

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Description

Introduction

In environmental and civil engineering, surface-mounted hemispherical bluff bodies are commonly used as architectural design elements. They appear in various applications such as depicted in Fig. 2. Although the hemisphere represents a simple geometry, it exhibits a rather complicated flow field including complex flow patterns. They can be roughly classified into an upstream horseshoe vortex system and a recirculation area with trailing vortices in the wake region. Furthermore, full-scale structures are often exposed to turbulent boundary layers, which increases the complexity of the flow field. The specific flow problem of a wall-mounted hemisphere was studied since the 1940s. However, due to its complexity, most of the contributions focus on a certain part of the flow. The present study published in Wood et al. (2016) includes experimental investigations and large-eddy simulations (LES) to characterize the whole three-dimensional flow field around a surface-mounted smooth hemisphere in a turbulent boundary layer at Re = 50,000.

UFR3-33 dome examples.png

Fig. 2: Examples of hemispherical domes in modern civil engineering.

Review of previous work

The following brief literature review is extracted from Wood et al. (2016). A full version can be found in the paper.

Previous experimental investigations

Beginning with an analysis based on the pressure distribution the first experiments were carried out by Jacobs (1938), Maher (1965) and later on by Taylor (1992) to mention only a few. They showed Reynolds number independency above a critical Re number and turbulence intensity. Another experiment by Taniguchi et al. (1982) found a relationship between the approaching boundary layer thickness and the aerodynamic forces acting on the hemisphere. Toy et al. (1983) and Savory and Toy (1986) investigated the flow past a hemispherical dome using hot-wire and pulsed-wire anemometers and brought a deeper insight into the complex flow structures occurring in the near-wake regime. Another experiment by Savory and Toy (1988) focused on the separation of the shear layer in the flow around hemispheres at a sub-critical Reynolds number of Re = 140,000. The study included different turbulent boundary layers classified in thin, smooth and rough boundary layers depending on the thickness and the turbulence intensity. A profound discussion provided a deeper understanding of the distribution of the turbulent shear stress and intensity in the wake regime. Additionally, a representative illustration of the flow field characteristics was provided. The outcome of both studies (Savory and Toy 1986, 1988) is often used as reference for experimental and numerical examinations.

Visualizations of the vortical flow structures were conducted by Tamai et al. (1987). The experiments allowed to visualize the complex vortical structures by injecting dye into the water channel. Moreover, the frequencies of the vortex formation and shedding from the separation area were recorded. Bennington (2004) examined various roughness elements and their associated effects on the turbulent boundary layer. Among the chosen elements, a hemispherical obstacle was analyzed in detail concerning statistics of the Reynolds stresses, the turbulent kinetic energy and even the triple correlations. Further visualization experiments were conducted by Yaghoubi (1991). They comprised a detailed visualization of the flow pattern around grouped hemispheres in a wind tunnel. The motivation was to achieve a deeper understanding of the flow field and the associated effects of natural ventilation of domed structures.

Previous numerical investigations

Apart from experimental investigations numerical simulations were carried out to provide enhanced insight into the flow. An early study was conducted by Tamura et al. (1990) without applying any turbulence model. The focus of the simulations lay on the visualization of the unsteady flow pattern and the time-averaged surface pressure distribution. A fundamental numerical study was carried out by Manhart (1998) using large-eddy simulation to receive more detailed information about the vortical structures. The Cartesian grid combined with the immersed boundary technique led to an artificial surface roughness on the contour of the hemisphere. The results were therefore compared with the experiments of Savory and Toy (1986, 1988) for a rough hemisphere. Another comparison of numerical and experimental data was made by Meroney et al. (2002). The wind load distributions on smooth, rough and dual domes in the shape of hemispherical caps were examined. Several RANS turbulence models were used delivering similar results. Recently, Kharoua and Khezzar (2013) performed a LES on a hemisphere with a rough and smooth surface comparing the results with the experiment of Savory and Toy (1986). A specialized approach to model the surface roughness was presented. The results of the LES allowed the visualization of instantaneous three-dimensional flow patterns illustrating the complex interaction of vortical structures in the close vicinity of the hemisphere.

Previous complementary experimental and numerical investigations

A combined experimental and numerical study was accomplished by Tavakol et al. (2010). A hemisphere was immersed in two turbulent boundary layers of different thickness. Velocity distributions and turbulence intensities were presented for the streamwise and the spanwise directions in the recirculation zone. A further velocity measurement was carried out for the area close to the front of the hemisphere investigating the horseshoe vortex that leads to a strong backflow in the near-wall region. The numerical investigation relied on the RNG k-ε turbulence model (Yakhot et al., 1992). The inflow conditions of the simulation were generated by implying the time-averaged data of the corresponding hot-wire measurements. The turbulence intensity at the inlet was also taken from the measurements. The results showed overall good agreement with the experimental data. Recently, Tavakol et al. (2014) presented a yet deeper investigation of the hemisphere flow using LES. Based on the earlier study (Tavakol et al., 2010) the main focus was to highlight the superior results of the applied LES compared to the previously performed RANS simulations. The study included a thin turbulent boundary layer δ/D = 0.15 as inflow condition. For a realistic inlet velocity distribution including fluctuations, a turbulence inflow generator based on the method of Sergent (2002) was applied. The LES results showed excellent agreement with the measurements. An updated comparison between the previous study (Tavakol et al., 2015) with the current data revealed the shortcomings of the RNG k-ε model. A presentation of time-averaged data focuses on the streamline visualization and surface pressure distribution. Unfortunately, the study does not present statistical data of the velocity field or the Reynolds stresses.

Choice of test case

The literature review presented indicates that a surface-mounted hemisphere placed in a turbulent boundary layer exhibits a very complex flow field. The key aspect of most studies listed above often remains on one specific issue such as the recirculation area or the pressure distribution. Just a few studies contain general characteristics of the flow including complementary numerical and experimental investigations.


The present UFR is based on the experimental LDA and numerical LES study of Wood et al. (2016) which focuses on the following objectives:

  • It provides a comprehensive view of the flow field past a hemispherical object immersed in a turbulent boundary layer at Re = 50,000 with the help of experimental and numerical investigations: All relevant regions of the flow field (horseshoe vortex system, recirculation area and wake) are studied in detail including unsteady characteristics such as vortex shedding and related spectral analysis.
  • To avoid uncertainties in the numerical model, the surface of the hemisphere is assumed to be smooth. The experimental data are recorded based on a very smooth aluminum model with low engineering tolerances which are expected to minimize possible influences of surface roughness during the measurements.
  • The characteristics of the oncoming turbulent boundary layer are taken from the wind tunnel measurements and transferred to the numerical domain. This matching ensures the comparability between the experiment and the numerical simulation, focusing on the mean velocity profile and the turbulent fluctuations of the boundary layer. The numerical simulations use a synthetic turbulent inflow generator (STIG) approach to mimic the turbulent fluctuations of the boundary layer. That is found to be an important issue, since the turbulent fluctuations have a significant impact on the overall flow-field charateristics such as reattachment and separation points.
  • The time-averaged flow field is analyzed in detail, focussing on the mean velocity distribution in the symmetry plane. Furthermore, the corresponding Reynolds stresses of the flow field past the hemisphere are analyzed in detail including a comparison between the experimental measurements and the numerical simulation data.
  • The effect of well-known and often applied subgid-scale (SGS) models is highlighted to present their influence on the results.
  • The test case with complementary experimental and numerical data for the surface-mounted hemisphere flow offers a novel benchmark for the evaluation and validation of numerical schemes or new turbulence models.
  • Finally, the investigation of the flow past the rigid structure is the first step towards an investigation of the coupled fluid-structure interaction of the flow around a flexible membranous structure. Such as study is intended for the near future.



Contributed by: Jens Nikolas Wood, Guillaume De Nayer, Stephan Schmidt, Michael Breuer — Helmut-Schmidt Universität Hamburg

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