Julien Weiss

Method for the determination of frequency response and signal to noise ratio for constant-temperature hot-wire anemometers

Determination of the frequency response of constant-temperature hot-wire anemometers is needed for measurements of high frequency turbulent fluctuations or when the bridge cannot be well adjusted because of too short a testing time. A method is proposed to determine the complete transfer function of a constant-temperature anemometer bridge in several milliseconds by means of an electrical test. The frequency response is used to perform postcorrection of the data, which enables the measurement of turbulent quantities at frequencies higher than the cut-off frequency of the system, when the bridge adjustment is not optimum. The technique, which is tested in the free stream of a supersonic wind tunnel at M=2.5, also enables a accurate estimation of the signal to noise ratio.

Constant temperature hot-wire measurements in a short duration supersonic wind tunnel

A constant temperature hot-wire anemometer enabling automatic rapid scanning of the wire overheat was built to perform free stream disturbance measurements in the shock wind tunnel of the Institute for Aerodynamics and Gasdynamics at Stuttgart University. It is shown that such a system brings real advantages in terms of testing time. The change of bridge dynamic behaviour with wire temperature is taken into account by measuring the bridge frequency response with a very fast electrical test and postprocessing the data. The method of operation is validated in a supersonic suck down wind tunnel and a comparison with a commercial constant temperature bridge shows good agreement. Results of free stream disturbance measurements in a short duration supersonic wind tunnel of 120ms testing time are presented.

A new wind tunnel for the study of pressure-induced separating and reattaching flows

The design, construction, and validation of a new academic wind tunnel is described in detail. The wind tunnel is of a classical, blow-down type and generates a pressure-induced, turbulent separation bubble on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness just upstream of separation, is Re θ ≃ 5,000 at a free-stream velocity of U ref = 25ms −1 . The length of the separation bubble is estimated at 0°42 ± 0°02m by three different methods. Results of a numerical simulation demonstrate the absence of flow separation in the wind-tunnel contraction. This results in a turbulence level of about 0·05% in the test section. Oil-film visualisation experiments show that the flow near the wall is strongly three-dimensional in the recirculating region and that the topology of the limiting streamlines is consistent with experiments performed on configurations with fixed separation. Finally, spatial variations of the forward-flow fraction have been documented using a thermal-tuft probe and are shown to compare well with the results of the oil-film visualisation.

Unsteady Behavior of a Pressure-Induced Turbulent Separation Bubble

Wall static-pressure and longitudinal-velocity fluctuations are measured in a pressure-induced turbulent separation bubble generated on a flat test surface by a combination of adverse and favorable pressure gradients. The Reynolds number, based on momentum thickness upstream of separation, is Reθ≃5000 at a free-stream velocity of Uref=25  m/s. The results indicate that the flow is characterized by two separate time-dependent phenomena: a low-frequency mode, with a Strouhal number St1≃0.01, which is related to a global “breathing” motion (i.e., contraction/expansion) of the separation bubble, and a higher-frequency mode, with a Strouhal number St2≃0.35, which is linked to the roll-up of vortical structures in the shear layer above the recirculating region and their shedding downstream of the bubble. These two phenomena are reminiscent of the “flapping” and “shedding” modes observed in fixed-separation experiments, though their normalized frequencies are different. The breathing mode is also shown to be str...

Lead-resistance effects in a constant voltage anemometer

Two effects of the lead resistances connecting the hot wire to a constant voltage anemometer (CVA) were analyzed and tested: one concerns the change in the sensitivity coefficient relating the anemometer output to velocity or temperature fluctuations, and the other the time constant of the hot wire determined by an in situ square-wave test technique. Small perturbations were assumed in both cases. The CVA output sensitivity was found to be reduced and the time constant increased with the lead resistance. Explicit formulas which involve the lead resistance, the cold wire resistance, and the wire overheat, as well as some characteristics of the CVA circuit, were established to take into account these effects. In the ranges tested, each effect can individually introduce as much as 10% error. Product of the two governs the overall response for the CVA. However, because the two effects change in opposite directions, interestingly, variation in the net response from their product is minimized. This feature may ...

Electronic noise in a constant voltage anemometer

The electronic noise and the signal-to-noise ratio in a constant voltage anemometer (CVA) are analyzed in terms of the main constitutive elements of the circuit. It is shown that the output voltage due to electronic noise decreases with the wire resistance, permitting one to know the noise upper limit by using the results of the unheated wire. The noise power spectrum increases at high frequencies as f2, like in other anemometers, because of the need to compensate for the thermal lag of the hot wire, thus leading to a reduced signal-to-noise ratio at high frequencies. Explicit formulas are given in terms of wire, CVA, and flow quantities. Measurements of electronic noise in a CVA prototype confirm the theoretical analysis and illustrate some interesting issues concerning measurements of noise and low levels of flow fluctuations.

Effect of bridge imbalance on the estimation of sensitivity coefficients for constant-temperature hot-wire anemometers

The Wheatstone bridge in which the hot wire is embedded in normal constant-temperature hot-wire anemometers is never perfectly balanced, due to the finite value of the amplifier offset. In this paper we investigate the influence of this imbalance on the estimation of sensitivity coefficients for velocity and temperature fluctuations. The analysis is made using the anemometer model developed by Perry and Morrison (1971 J. Fluid Mech. 47 577). It is shown that the absolute value of both non-dimensional sensitivity coefficients is always lower than in the ideal case of an anemometer which maintains the bridge perfectly balanced. The degree of discrepancy increases with increasing offset voltage and decreasing overheat ratio. However, it is shown that a direct calibration procedure takes this effect into account, thus ensuring an accurate determination of the sensitivity coefficients of the anemometer in practice. Experimental evidence confirming the theoretical results is presented.

Numerical investigation of wall mounting effects in semi-span wind-tunnel tests

In this paper, the effects of installing an aircraft half-model on a sidewall of a wind tunnel are investigated. Three-dimensional RANS computations are performed on a geometry representative of an actual modern widebody airliner. The influence of a non-metric spacer, introduced between the half-model and the sidewall, is studied. The study focuses on the effects of the spacer presence on the lift coefficient and the pressure distribution around the model. Computational results show a significant change of the flow field in the case of the half-span installation compared to the full-span case. Unlike several previous investigations showing that the lift usually increases with increasing spacer height, the present study shows that the influence of the spacer is strongly dependent on the Mach number and the angle of attack. It is also found that changing the spacer height may have positive impact on the correlation between semi-span and full-span results for the lift coefficient at specific Mach numbers and angles of attack, while also negatively affecting the correlations of the same coefficient for other flow conditions.

Investigation of nonlinear effects in constant-temperature anemometers

The nonlinear behavior of constant-temperature anemometers is investigated experimentally and numerically for three commercially available anemometers. The experiments are performed in the potential core of a subsonic jet by injecting electrical signals of varying amplitudes and frequencies in the hot wire and recording the anemometer output signals. The numerical model is based on Freymuth’s theoretical analysis (1977 J. Phys. E: Sci. Instrum. 10 705–10). The two approaches are in good agreement, which demonstrates the validity of Freymuth’s nonlinear theory of constant-temperature anemometers. The results confirm that significant errors are made in the third-order turbulence moment, also called the skewness factor, when the amplitude of velocity fluctuations is large and their frequency is not small compared to the cut-off frequency of the system.

Constant-Temperature and Constant-Voltage Anemometer Use in a Mach 2.5 Flow.

TECHNICAL NOTES are short manuscripts describing new developments or important results of a preliminary nature. These Notes cannot exceed six manuscript pages and three figures; a page of text may be substituted for a figure and vice versa. After informal review by the editors, they may be published within a few months of the date of receipt. Style requirements are the same as for regular contributions (see inside back cover).