Bed-mounted electro magnetic meters: Assessment of the (missing) technical parameters

Flow measurements in Urban Drainage Systems (UDS) are essential for pollution control and system management. Since the accuracy of, today the most popular, Acoustic Doppler Velocimeters is impeded by several factors, this research is focused on the alternative, or a supplemental, Electro-Magnetic Velocity (EMV) meters. EMV meters are more robust and can provide accurate low flow measurements, even when covered with porous sediment. However, the downside of EMV is the small control volume (CV) where the flow velocities are integrated in a non-linear manner to obtain a single, one-dimensional measured velocity. For a better understanding of the sensor output and measured mean flow velocity with quantified uncertainty, it is necessary to determine the size of the CV and to understand the non-linear integration principle within the CV. Valuable technical parameters, needed for describing these EMV properties, are typically not provided by the manufacturers. Fundamentally, they could be defined with the magnetic field and “virtual” current distributions. To allow for a more practical interpretation of the EMV operating principle, a simplified model of an EMV sensor is proposed here. The suggested model describes the EMV operating principle with only two technical parameters, one-dimensional weighting function $w$ and the reach of the CV, the ${\tau }_{max}$. Furthermore, a methodology is proposed for defining these two parameters, using two lab flume experiments. The first one is focused on the investigation of the EMV output, when the EMV is covered by the porous sediment with different depths. The second experiment involves the determination of the longitudinal velocity distribution within the lab flume and the CV of the EMV meter. A backward analysis is suggested to formulate a minimization problem, from which the unknown technical parameters are assessed. The proposed procedure was applied on the examined Flat DC-2 EMV meter. Derived one-dimensional weighting function $w$ exponentially drops with the distance from the electrodes, while the reach of the CV was found to be ${\tau }_{max}$ = 8.7 cm. These parameters, and the simplified model, were validated against the EMV outputs acquired in the lab flume, without sediment presence.     

A pre-verifiable calibration model of wall shear stress thermal sensor driven by constant current

This paper presents a pre-verifiable calibration model, which can accurately characterize the measurement behavior of the flexible wall shear stress thermal sensor in constant current drive mode. In this calibration model, the fitting parameter $a$ can be pre-verified with $I{R}_a$, which provides a method to verify the accuracy of calibration instruments and data. In repetitive experiments, the average related normalized standard deviation ${\epsilon }_{\tau }$ is 1.26%, which indicates that our calibrations are reliable. And the relative deviations $\eta$ between $a$ and $I{R}_a$ are below 1.15% in all experiments, which indicates that the calibration model can pre-verify the reliability of calibration instruments and data among different sensors and different drive currents.     

Similarity of the asymmetric swirl generator and a double bend in the near-field range

An asymmetric swirl generator (ASG) is considered to replace the current swirl generator in the upcoming 2020 revision of the standard for heat meters EN 1434. While recent studies have shown its superiority with respect to a reproducible representation of the double bend out-of-plane (DB) flow disturbance in the far field, there are still open questions regarding the similarity in the near-field range and the determination of an optimum testing distance. In this paper, we examine the performance of an ASG in the potential testing range and investigate an increase of the segmental orifice plate. Laser Doppler anemometry (LDA) measurements downstream of the ASGs and a DB carried out at Reynolds numbers ($Re$) of $5\times {10}^4$ and $5\times {10}^5$ are evaluated based on a comparison of the flow patterns and a quantitative analysis by means of performance indicators. The results indicate, that the original version of the ASG does not reach the level of asymmetry and swirl provided by the DB in the near-field range. An increase of the orifice height yields higher swirl and asymmetry, hence, a better similarity of the flow characteristics. A maximum degree of resemblance with the crescent-shaped velocity patterns of the DB is found seven diameters downstream of the modified ASG. In contrast, a testing distance of two diameters or less does not represent the DB, as the early flow development bears the traces of the disturber's geometry. The results may be considered for the implementation of the test procedure in EN 1434 and the next revision of the standards for water meters.     

Velocity measurements in developing narrow open-channel flows with high free-stream turbulence: Acoustic Doppler Velocimetry (ADV) vs Laser Doppler Anemometry (LDA)

A developing narrow open-channel flow has been investigated using Acoustic Doppler Velocimetry (ADV) and Laser Doppler Anemometry (LDA). The objectives were to first characterize the flow environment with the LDA system alone, then quantify the intrusion effect of the ADV sensor immersion, and finally compare ADV-LDA measurements. The main features of the flow have been described. The turbulence levels measured in the outer flow region are high and almost isotropic due to the specificities of the flow (3D, narrow, developing). This contributes to a flattening of the mean streamwise velocity profile in this region. The intrusion effect of the ADV sensor is found to be dependent on Froude number ($Fr=U_0/\sqrt{gH}$ with $U_0$ the discharge velocity, $H$ the flow depth, and $g$ the gravity acceleration). Vertical flow below the sensor is amplified while the streamwise component of the flow is enhanced for “low” Fr ($\le 0.6$) and reduced for “high” Fr ($\ge 1.1$). On the other hand, turbulence quantities are not affected by the sensor presence. Compared to the LDA, the ADV is shown to underestimate the mean flow and turbulence intensities, while not affecting Reynolds shear stress measurements. The underestimation of the turbulence intensities can be attributed to the lower sampling rate and larger sampling volume of the ADV, but the underestimations of the mean velocities are more likely linked to a constant bias that the ADV seems to have or to some type of ADV-intrinsic noise. Some implications for practical application are discussed.     

An anomalous method of using a camera in determining the velocity distribution within a boundary layer

The current study proposes a cost-effective technique wherein a camera can be used anomalously in tandem with the working of a pitot tube in determining the velocity distribution within a boundary layer. The technique involves the traversal of a pitot tube at a uniform velocity inside the width of boundary layer and simultaneously imaging the change in meniscus location of liquid column in the corresponding manometer, inside a wind tunnel set-up. The study aims in establishing a relation between the traversal velocity and the free stream velocity. The proposed approach holds the advantage of providing velocity values at equally spaced locations within the boundary layer, with the number of values depending on the velocity of retraction (traversal away from the surface) as well as the frame rate of recording. Experiments were performed at different combinations of retraction velocity ($V_r$) and Fan Speed for establishing the method. An optimum velocity at which the pitot tube should be retracted was determined based on convergence of trend of the variation in instantaneous meniscus locations to a standard reference trend. Results show that dimensionless optimum velocity (${\overline{V}}_o$) defined as the optimum ratio between tunnel velocity ($U_t$) and $V_r$ is related to the Fan Speed (FS) by ${\overline{V}}_o\propto$ FS${}^{0.6}$. Additionally, with regard to the manual traversing performed, the probability density function plots show that traversing manually at $\overline{V}\gg {\overline{V}}_o$ introduces higher non-uniformity in the results, thereby preferable to perform the traversal closer to values of ${\overline{V}}_o$. Using the results obtained from the developed imaging technique, the velocity variation within the boundary layer was studied. The technique was successful in precisely locating the prominent points inside a boundary layer for a particular flow condition. Finally as a case study, the imaging approach was also utilized in determining the effect of roughness change across the seam of a cricket ball on its aerodynamics. This case study also shows that the technique is successful in obtaining the flow characteristics of a general boundary layer as well as a flow separation case.     

Flow measurement using a triangular broad crested weir theory and experimental validation

The self-cleaning and semi-modular triangular broad-crested weir without crest height was firstly subjected to a rigorous theory. The main objective was to establish the discharge relationship as well as that of the resulting discharge coefficient. For this, both energy equation and momentum equation applied between two judiciously chosen sections were necessary and proved to be essential. Contrary to previous studies related to flow metering, the relationship governing the flow rate was established by taking into account the approach flow velocity. Secondarily, the device was subjected to an intense experimental program to confirm the validity of the proposed theoretical relationships. It was observed an excellent agreement between the experimental and theoretical values of the flow rate. It has been found that the experimental and theoretical flow rates are related by a linear relationship such that $Q_{Exp}=1.0057Q_{Th}$. The constant clearly indicates that the flow rate theoretical formula only needs a slight correction. The theoretical stage-discharge formula was then very accurate even no calibration parameter was employed. The theoretical development has shown that the discharge coefficient C_d only depends on the dimensionless parameter M₁ that reflects the effect of the contraction of the cross-section of the approach channel. The variation curve of C_d(M₁) showed that C_d increases in the range [0.233; 0.277] with the increase in M₁.