Jean Lemay

Gurney Flap Scaling for Optimum Lift-to-Drag Ratio

This note aims at providing evidence that there exists a flow-based scaling for the Gurney flap heights that yield an increase in lift-to-drag performance compared with the baseline airfoil at the same angle of attack (beneficial Gurney flaps). The results presented here, support this statement and further suggest that the boundary-layer thickness δ, measured at the trailing edge on the pressure side of the baseline airfoil, is not only an appropriate flow-based normalization for the flap height but is also a proper order of magnitude for the flap height providing the largest increase in the lift-to-drag ratio (optimum Gurney flap)

Self-Similarity in the Outer Region of Adverse-Pressure-Gradient Turbulent Boundary Layers

This paper presents a consistent theory of self-similarity and of equilibrium in the outer region of turbulent boundary layers that explains recent experimental findings on the subject, including new ones presented here. The theory is first presented in a general form where the outer scales are left unspecified and it is not assumed that the mean velocity defect and the Reynolds stresses share a common velocity scale. It is shown that the main results of the traditional similarity theory remain valid even in this case. Common outer scaling with the Zagarola-Smits length and velocity scales is then chosen. A new pressure gradient parameter is introduced to characterize the local effect of the pressure gradient in all flow conditions including strong adverse-pressure-gradient conditions. By analyzing several adverse-pressure-gradient flow cases, it is shown that self-similarity of the mean velocity defect profile is reached in all cases in localized but significant flow regions. The same is, however, not true of the Reynolds stress profiles. In agreement with the similarity analysis, the self-similar velocity defect profile is found to be a function of the pressure gradient and most flows studied here are only in an approximate state of equilibrium in the region of self-similar defect profiles despite the excellent collapse of the profiles.

Development of a parallel plate flow chamber for studying cell behavior under pulsatile flow.

&NA; The design of a new parallel plate perfusion chamber for cell behavior studies involving pulsatile flowrates is presented. It was based on fluid mechanical considerations to ensure a region of regular and uniform shear stress at the wall. A numeric solution of the flow was performed to study the effect of pulsating flow on the entrance length. Dye injection investigations in the chamber showed laminar and uniform flow in the culture region under steady state conditions. ASAIO Journal 1995;41:876‐883.

Some Similarities Between Turbulent Boundary Layers Manipulated by Thin and Thick Flat Plate Manipulators

This paper presents a dual study performed in order to compare the influences of both thin and thick flat plate manipulators on turbulent boundary layers. Mainly, the same features are observed for both configurations. In the thick ribbon configuration, a larger wake with a stronger momentum defect induces a quicker initial Cf reduction and a longer relaxation length. Broadly, the components of the Reynolds stress tensor show the same evolution. Moreover, the structures of both manipulated boundary layers are quite comparable. Large scale suppressions throughout the layer and the injection of small scale structures by the ribbon wake are observed on the u´ spectra. The space-time correlation coefficient contours reveal that the longitudinal and vertical integral scales are dramatically reduced by the manipulation, whilst the spanwise integral scale is almost unchanged. However, the spanwise organization of the structures of the manipulated boundary layer is quite modified. The analysis of the spectra and space-time correlation coefficient contours reveals that, at a given streamwise position, the perturbation induced by the thicker ribbon is at an earlier stage in its evolution than the other one. Finally, from a structural point of view, the manipulated boundary layers have a surprisingly long memory.

Turbulent kinetic energy balance in a LEBU modified turbulent boundary layer

An experimental study of the turbulent kinetic energy balance is performed in a LEBU manipulated turbulent boundary layer. The estimation of almost all the terms of the k-equation is obtained by hot wire anemometry. Near the manipulating device, strong alterations are observed, when compared with the natural conditions. The wake of the manipulator imposes two distinct zones. The lower part is characterized by negligible production compensated by diffusion, while, in the outer part, a large excess of production is balanced by diffusion and dissipation. The excess of dissipation rapidly vanishes downstream. The relaxation process is slower for production and diffusion.

Correction of cold-wire response for mean temperature dissipation rate measurements

Abstract This work is aimed at improving dissipation rate measurements using cold-wire anemometry. The mean dissipation rate of the temperature variance is measured on the axis of a heated turbulent round jet. Measurements are performed with a constant current anemometer (CCA) operating fine Pt–10%Rh wires at very low overheat. The CCA developed for this purpose uses a current injection method in order to estimate the time constant of the wire. In the first part of the paper, it is shown that the time constants obtained for two wire diameters ( d =1.2 and 0.58 μm) compare well with those measured at the same time using two other methods (laser excitation and pulsed wire method). Moreover, for these two wires, the estimated time constants are in good agreement with those obtained from theory. In the second part of the paper, a compensation procedure, involving post-processing filtering, is developed in order to improve the frequency response of the cold-wire probes. Measurements on the jet axis (Re D =16 500 , Re λ =167) show that the frequency response of the 1.2 μm wire is indeed significantly improved after compensation. The spectrum of the compensated signal for the 1.2 μm wire compares fairly well with that for the 0.58 μm wire. The results also indicate that the compensation should be applied when the cut-off frequency of the cold-wire f C is less than about 2 f K , where f K is the Kolmogorov frequency. When f C ≈0.6 f K , the compensation can reduce the error in the estimated mean dissipation rate by more than 20%. When f C =2 f K , the reduction is about 5%.

Use of the Wake of a Small Cylinder to Control Unsteady Loads on a Circular Cylinder

Simultaneous measurements of lift and drag forces have been performed in order to study passive control of unsteady loads induced on a circular cylinder. For this purpose, an aerodynamic balance has been developed. The balance, developed for a cylinder of 25.4 mm in diameter, was designed to operate in the subcritical regime (Re=32000). This instrument is characterized by its sensitive element that forms a small central part of the cylinder. The static and dynamic calibrations of the balance show the appropriateness of the present design. Moreover, qualification experiments carried out with a single cylinder gave results (mean and rms values of the lift and drag coefficients) that are in good agreement with those found in the literature. The purpose of this paper is to present a passive control experiment performed by means of the wake of a smaller cylinder interacting with a larger one. Firstly, a parametric study was performed by varying the following: i) the diameterdS of the small cylinder for one large cylinder diameterd (7 values in the range 0.047≤dS/d≤0.125); ii) the center-to-center spacingS/d (11 values in the range 1.375≤S/d≤2.5); and iii) the stagger anglea(0≤a≤90 with a fine angular stepDa fora≤15). A maximum mean drag reduction of about 48% is achieved. Ata=4 to 8, one can observe a peak of mean lift coefficient. Then unsteady fluid forces, vortex shedding frequency and flow pattern were systematically investigated for the small cylinder having a diameterdS=2.4 mm (0.094d). Reductions of 78% and 56% of the rms lift and drag fluctuations respectively were obtained with the small cylinder placed at a slight stagger angle in the range 6<a<9. This leads to an instantaneous force vector that exhibits more steadiness both in angle and amplitude. Moreover, at these stagger angles, the energy of the lift fluctuations at the shedding frequency is significantly reduced compared to the single cylinder case.

Development of a flow simulator to study haemodynamic behaviour of natural and artificial blood vessels under physiologic flow conditions

A new computer-controlled flow simulator has been designed to study the haemodynamic behaviour of natural and artificial blood vessels under physiologic flow conditions. The simulator can generate well characterized and fully developed laminar flow properties. It includes a unique perfusion case that imposes an axial tension on the vessel segment, and a commercial programmable pump to reproduce pulsatile flow rates. Response to high frequency commands was greatly attenuated and displayed a frequency dependent phase angle. Thus, for complex pulsating flow rates containing different frequency components, the system response was significantly distinct from the command. To reproduce physiologic waveforms, the transfer function of the whole system was determined for different amplitudes and frequencies of flow rate excitations. Each input command was compared to the measured flow rate, and the values of the gain and phase angle were evaluated. If the desired flow rate was composed of a sum of n sine wave components, each has a frequency fj and an amplitude Aj, a corrected command signal was then reconstructed by amplifying the attenuated components and advancing those lagged in time. The corrected signal was finally applied as the new command to the pump. The results showed an excellent agreement with physiologic waveforms. Examples of different pulsatile flow experiments to investigate the effects of frequency, pressure, and wall elasticity are presented.

Improvement of the Response of a Fine Cold-Wire for the Measurement of High-Frequency Temperature Fluctuations

A number of investigations has been devoted to the determination of wire time constants (cf. the review in Brunn [1]). Nevertheless, one must notice that a limited number of contributions has been addressing this problem for very fine wires (diameter < 1 µm); LaRue et al. [2], Fiedler [4], Antonia et al. [3] and Paranthoen et al. [5] are among the contributors to that subject. For wire diameters larger than 1 µm, most of the results reported in the literature are in good agreement with the theoretical values. However, for the finest wires (0.63 and 0.25 µm) the forementioned authors have measured time constants larger than the theoretical ones. In the present paper, the time constant of 1 and 0.5 µm diameter wires are obtained via: 1) a square-wave current injection producing a Joule effect heating 2) a chopped laser beam producing a radiant square-wave heating, and 3) a tracer of heated air convected in the wake of a 5 µm wire excited with a voltage pulse. These time constants are measured as a function of the cooling velocity, the wire being exposed to the potential core of an axisymmetric air jet. Once the time constant of a wire is determined, we compensate its response by applying a numerical post-processing technique.

Analysis of the Wake of an Outer Layer Manipulator

Among the different ways of manipulating turbulent boundary layers to obtain drag reduction, external manipulation (blades or aerofoils profiles) has been intensively investigated. If the Cf reduction downstream of the manipulator is somewhat comparable whatever the configuration, it seems that the net drag reduction that can be achieved is strongly related to the manipulator by itself, or more precisely to the interaction between the turbulent structures of the boundary layer and the manipulator. For example, the unsteadiness of the large scale structures of the boundary layer or Large Scale Motions (L.S.M.) as called by Falco /1/ may create separated regions on the manipulator and by this way increase dramatically its drag. On the other hand, the incoming L.S.M. may modify the coupling between the wake structures of the manipulator and the surrounding flow. In this context, we first analyse the coherence of longituninal velocity in the wake; second, a slight heating of the manipulator is used in order to get a passive marker of the flow passed on it.