EXP 1-2 Measurement Quantities and Techniques

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Front Page

Introduction

Review of experimental studies

Description

Experimental Set Up

Measurement Quantities and Techniques

Data Quality and Accuracy

Measurement Data and Results

Measurement quantities

Since the two instantaneous velocity components (longitudinal, $u$, and vertical, $w$, for the top openings and longitudinal, $u$, and lateral, $v$, for the lateral openings) and the pollutant concentration ($c$) were measured simultaneously at each given point, the following quantities normalised to the reference flow velocity $U_{ref}$ (which was the freestream velocity of the wind tunnel) were calculated from 120-s time series and are listed in Measuring data and Results:

• The mean longitudinal ($U$), vertical ($W$) and lateral ($V$) velocities.
• The standard deviations (e.g. $U_{std}$), kurtosis (e.g. $U_{kurt}$) and skewness e.g. $U_{kurt}$) of each the given (e.g. $U$) velocity component.
• The intensity of turbulence of the given velocity (e.g. $I_{U}$)
• The mean vertical (${\overline {u'w'}}$) and lateral (${\overline {u'w'}}$) momentum fluxes.
• The mean dimensionless concentration (${\frac {CU_{ref}HL}{Q}}$, where $C$ is the mean concentration in ppm, $H$ is the reference building height, $L$ is the length of the line source and $Q$ is the volumetric flow of the pollutant - ethane).

The description of the qunatities and the locations where these quantities were measured in the street canyons can be found in Description of the study test case.

Measurement techniques

When measuring the turbulent pollution fluxes at the openings of the street canyons, two velocity components were measured simultaneously and point by point. For the upper openings, the longitudinal and vertical components were measured, and for the lateral openings, the longitudinal and lateral components were measured. This measurement was made with a combination of LDA (Laser Doppler Anemometry) and FFID (Fast-Response Flame Ionisation Detector, HFR400, Cambustion Ltd.).

To ensure proximity between the LDA measurement volume and the inlet of the FFID sampling tube, the LDA and FFID probes were mounted together on a 3D traverse system. The position of the inlet of the FFID sampling tube was carefully adjusted to be 1 mm above, 1 mm behind and 1 mm beside the centre of the LDA measuring volume (Fig. 3). Through various test measurements with different probe positions, we confirmed that the influence of the FFID sampling tube on the LDA measurement was negligible.

During the measurement campaign, the LDA sampling frequency was kept between 0.5 and 1 kHz, depending on the flow range investigated. The FFID sampling frequency was set to 0.5 kHz to achieve the desired response time of 2 ms. However, due to the physical characteristics of the FFID sampling tube (length of 200 mm and diameter of 1.2 mm), an average time delay of about 12 ms was obtained in contrast to the LDA. This time delay varied depending on the air density and the dynamic pressure at the sampling point. To obtain a more accurate individual FFID time delay, we used the maximum correlation coefficient between the time series of the velocity component and the concentration. This resulted in an adjusted time delay between 10 and 13 ms.

To account for the influence of the LDA seed particles (with a diameter of about 1 μm) on the FFID concentration measurements, a correction was applied during the FFID calibration process. The measurements of the background concentration of the LDA seed particles were performed separately and subtracted from the measured calibration gas without activating the line source.

Figure 3: Snapshot of the experimental setup for the point measurement of turbulent vertical pollution fluxes in the case of the oblique wind direction.

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Contributed by: Štěpán Nosek — Institute of Thermomechanics of the CAS, v. v. i.

Front Page

Introduction

Review of experimental studies

Description

Experimental Set Up

Measurement Quantities and Techniques

Data Quality and Accuracy

Measurement Data and Results

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