Rms Values Of Velocity Fluctuations Research Articles

Experimental investigation of the effects of length-to-diameter ratio of hot-wire sensor on the dynamic response to velocity fluctuations

This paper reconsidered the criterion on the length-to-diameter ratio (l/d) of the hot-wire sensor, $$l/d \ge 200$$ , for end-conduction effects on the dynamic response of constant-temperature hot-wire anemometer (CTA) to be neglected. Experiments were first conducted using a low-noise compact CTA with hot-wire sensors of l/d = 80, 120, 150 and 200 in a two-dimensional (2-D) circular cylinder wake at a Reynolds number Re = 77, at which a 2-D von Karman vortex street developed, allowing us to exclude spatial filtering and tangential velocity effects. The damping coefficient in the CTA bridge was tuned to 0.70 for all sensors to reduce possible influences of over- or under-damping on measurements in a frequency range up to 6 kHz. The stubs and prongs of probes consisted of identical materials and dimensions so that the thermal properties were considered to be identical for all probes. Careful comparisons of the amplitudes of the fundamental (1.9 kHz) and higher harmonic components of time-periodic wake vortices showed that the attenuation due to the end-conduction effect in the absence of the spatial filtering and damping coefficient effects was at most 2.5% in terms of the rms value of velocity fluctuations even for l/d = 80. The similar weak attenuation (about 2.6% for l/d = 80) was also observed in the measurements of low-frequency free-stream turbulence. In addition, effects of l/d on the yaw angle response of the sensor were examined in the free stream, revealing that the yaw angle ( $$\psi $$ ) variation of the measured velocity (both the mean and fluctuations) obeyed the cosine law for $$\psi \le 60^\circ $$ . The Champagne constant k was 0.12 for l/d = 200 and 0.2 even for l/d = 80 in the mean velocity measurement using the present probe with long stubs. On the other hand, the intensity of free stream turbulence measured using a $$45^\circ $$ -yawed sensor was significantly affected by the sensor’s l/d; the rms value of velocity fluctuation was attenuated by about 7.5% for l/d = 80.

Experimental study of a round jet impinging on a flat surface: Flow field and vortex characteristics in the wall jet

Impinging jets are widely used in cooling applications. Here, particle image velocimetry measurements were performed to study the flow field (focusing on the wall jet) and vortex characteristics of a round air jet, impinging on a flat surface at three Reynolds numbers, Re = 1,300, 6,260 and 12,354 (based on nozzle diameter, D, and jet exit velocity), and stand-off distance, 4.75D. In the wall jet, self-similarity (outer layer scaling) of the mean radial velocity, rms values of velocity fluctuations and Reynolds shear stress was obtained for Re = 12,354. At Re = 1,300, impinging primary vortices generated highly coherent primary-secondary vortex pairs that were convected along the wall. In contrast, at the two highest Re, primary vortices broke-up into small-scale structures prior to impingement and vortex pairs were only revealed after conditionally averaging the data. Their strengths, areas and numbers were analyzed using the instantaneous swirling strength and vorticity distributions. Primary vortex strength peaked at break-up or impingement (Re = 1,300) and reduced during interaction with the secondary vortex. Analyzing the different contributions to the averaged vorticity equation revealed that stretching and realignment due to the mean flow always strengthened the vortices while turbulent diffusion mainly weakened them.

Experimental study on dependence of streak breakdown on disturbance nature

Instability and breakdown of a low-speed streak initially forced by disturbances with a frequency-spectrum similar to that of wall turbulence were examined experimentally. A single low-speed streak was generated using a small piece of screen set normal to a boundary-layer plate, and disturbances made by two loudspeakers were introduced through two small holes. Turbulent velocity fluctuations with various spanwise coherences were used as driving signals to two loudspeakers to see how strongly the streak breakdown depended on the nature of disturbances. Signals were deduced at two spanwise locations with distance Δz+= 24–149 in the buffer region of a turbulent boundary layer by using a pair of hot-wires. The correlation coefficient Ct of the two turbulent signals ranged from −0.11 to 0.56. Both sinuous and varicose modes were excited but the former became dominant downstream in all cases. Development of the disturbance excited was compared to those for anti-symmetric (Ct=−1) and symmetric (Ct=1) excitations with the same turbulent spectrum. The comparison showed that the magnitude of the sinuous instability mode excited was dependent on the initial disturbances, but not strongly for −1≤Ct≤0. That is, in terms of the rms value of velocity fluctuations, the magnitude of the sinuous instability mode for the initial disturbance with Ct∼0 was about 75% of that in the case of anti-symmetric forcing. Even for Ct∼+0.56, the magnitude of the sinuous mode excited was as large as 50% of that in the anti-symmetric case.

Analysis of conditional statistics obtained near the turbulent/non-turbulent interface of turbulent boundary layers

Direct numerical simulations of turbulent boundary layers (TBL) along a flat plate are used to study the properties of turbulent/non-turbulent (T/NT) interface of the TBL. The values of the momentum-thickness-based Reynolds numbers, Reθ, used for this study, are 500–2200. Analysis of the conditional statistics near the interface of the TBL shows that there are a small peak in the spanwise vorticity, and an associated small jump in streamwise velocity. It is shown that the interfacial layer has an inertia-viscous double structure which consists of a turbulent sublayer with a thickness of the order of the Taylor micro-scale and its outer boundary with a thickness of the order of the Kolmogorov length scale. The velocity jump near the T/NT interface of the TBL is of the order of the rms value of velocity fluctuations near the interface. Conditional cross correlations of the streamwise or the wall-normal velocity fluctuations change sharply across the interface, which are consistent with the blocking mechanism of the interface.

Measurements of pollen grain dispersal in still air and stationary, near homogeneous, isotropic turbulence

The dispersal of ragweed, pine and corn pollen as well as polystyrene spheres in still air and stationary, near homogeneous, isotropic turbulence (HIT) was investigated using high-speed, digital inline holographic cinematography enabling Lagrangian tracking of the particles. Mean still air settling velocities were similar as reported literature values. Small discrepancies were most likely related to species/size differences and water content of the grains. Near-HIT was generated by loudspeakers mounted on the corners of a 40 cm 3 chamber and the turbulent flow field at the center of the chamber was validated using stereoscopic Particle Image Velocimetry (PIV). Results showed near homogeneity and near isotropy with mean velocities 5–10 times smaller than the corresponding rms values of velocity fluctuations. The turbulent kinetic energy dissipation rate was determined from the PIV data sets and used to calculate the Kolmogorov scales and Taylor microscales. Experiments were carried out for two different loudspeaker amplifications corresponding to Taylor microscale Reynolds numbers, R λ =144 and 162, respectively. The mean settling velocity in turbulent conditions was in all cases higher than the corresponding still air value, the difference becoming smaller as particle Stokes numbers increased. For the present conditions, the still air particle settling velocity was lower than the rms values of air fluctuating velocities. As a result, dispersion was dominated by inertia and for a given R λ , particle fluctuating velocity autocorrelations fell more rapidly as the particle Stokes number decreased; corresponding particle diffusion coefficients also decreased. Transverse particle diffusion coefficients were lower than those in the direction of gravity in agreement with the continuity effect. Under the present range of experimental parameters, results showed that inertial particles (0.6<St<11) in highly turbulent conditions disperse more effectively than the air.

Flow measurement around a cycloidal propeller

An experimental investigation was conducted to study the flow around a cycloidal propeller. Flow fields were obtained using a particle image velocimetry system whose data acquisition was synchronized with the propeller’s angular position. The chord-based Reynolds number was Re c = u r c/υ = 1.4 × 104, where u r is the rotational velocity of the propeller and c is the chord length of the airfoil. Flow characteristics such as mean velocity, vorticity and the RMS value of velocity fluctuation were derived from the measurements. The results demonstrated the presence of a downwash around the propeller during the generation of lift. Detailed observations around each airfoil visualized distinct vortex shedding and reattaching flow at certain phase angles of the propeller.

Suction effects on turbulence flows over a dune bed

This experimental study investigates how suction affects turbulence flow over an immobile, two-dimensional triangular dune. A Laser Doppler Anemometer is used to measure the streamwise and vertical components of velocity over one dune length for three different suction rates at 0, 0.29% and 0.45%, respectively. The response of the mean flow and turbulence velocities to bed suction is explored in both a local sense, through comparison of the time-average velocity, rms values of velocity fluctuations and Reynolds shear stress profiles at five selected sections; and a spatial sense, by producing a set of contour plots over the dune length for all mean and turbulence parameters. The measurements reveal that suction causes the flow to adhere to the bed resulting in reduction of the separation length. The streamwise velocity profile becomes more uniform when compared to that without suction while suction always produces smaller or more negative vertical velocities in the near-bed region. The measurements also show that both the near-bed turbulence intensity and Reynolds shear stresses decrease with suction, which confirms that suction causes the flow within the boundary layer to be less turbulent. An interesting observation is that the flow does not respond to suction in the circulation zone just downstream of the dune crest.

Sensitivity of a Square Cylinder Wake to Forced Oscillations

The wake of a square cylinder at zero angle of incidence oscillating inline with the incoming stream has been experimentally studied. Measurement data are reported for Reynolds numbers of 170 and 355. The cylinder aspect ratio is set equal to 28 and a limited study at an aspect ratio of 16 has been carried out. The frequency of oscillation is varied around the Strouhal frequency of a stationary cylinder, and the amplitude of oscillation is 10–30% of the cylinder size. Spatial and temporal flow fields in the cylinder wake have been studied using particle image velocimetry and hot-wire anemometry, the former providing flow visualization images as well. A strong effect of forcing frequency is clearly seen in the near wake. With an increase in frequency, the recirculation length substantially reduces and diminishes the time-averaged drag coefficient. The time-averaged vorticity contours show that the large-scale vortices move closer to the cylinder. The rms values of velocity fluctuations increase in magnitude and cluster around the cylinder as well. The production of turbulent kinetic energy shows a similar trend as that of spanwise vorticity with the former showing greater asymmetry at both sides of the cylinder centerline. The instantaneous vorticity contours show that the length of the shear layer at separation decreases with increasing frequency. The effect of amplitude of oscillation on the flow details has been studied when the forcing frequency is kept equal to the vortex-shedding frequency of the stationary cylinder. An increase in amplitude diminishes the time-averaged drag coefficient. The peak value of rms velocity increases, and its location moves upstream. The length of the recirculation bubble decreases with amplitude. The reduction in drag coefficient with frequency and amplitude is broadly reproduced in experiments with the cylinder of lower aspect ratio.

Effects of Free-stream Turbulence on a Turbulent Boundary Layer (2nd Report, Effects of Grid Turbulence on a Convective Boundary Layer)

Effects of external turbulence in a free stream on a convective boundary layer over a flat boundary are investigated by means of laboratory experiments using a wind tunnel. Turbulence-generating grid is installed upstream of turbulent boundary layer to generate nearly isotropic, external turbulence. The bottom wall of the test section is heated and thus convective boundary layer with unstable thermal stratification (i.e., negative vertical temperature gradient) is developed. Instantaneous velocities, U and V, and instantaneous temperature T are measured simultaneously using a hot wire anemometry with X-probe and a constant current resistance thermometer. The results show that rms value of velocity fluctuations and Reynolds stress are strongly suppressed by the external grid turbulence as in the isothermal case. In the convective boundary layer, rms value of temperature fluctuation and vertical turbulent heat flux are also strongly suppressed by the grid turbulence. The power spectra of u, v and θ and their cospectra show that almost all scale motions are suppressed by the grid turbulence as in the isothermal case.

Reverse-Bifurcation in the Transition Process of Spherical Couette Flow.

In the spherical Couette flow with only an inner sphere rotating, relaminarizing flow in the laminar-turbulent transition process has been investigated by the introduction of infinitesimal disturbances from the external into the supercritical flow modes with a pair of toroidal Taylor-Gortler (TG) vortices. Normal and reverse bifurcations are discussed. The onset Reynolds numbers of toroidal TG vortices, spiral TG vortices and traveling waves as well as the onset Reynolds number of the relaminarization are influenced by the external disturbances, although the critical Re of the primary instability is not influenced. Influence of the external disturbances on rms values of velocity fluctuations is greater in the relaminarizing Re-region than in the lower Re-region. Fundamental frequencies of disturbances are not influenced by the external disturbances.

A bypass wake induced laminar/turbulent transition

The process of laminar to turbulent transition induced by a von Karman vortex street wake, was studied for the case of a flat plate boundary layer. The boundary layer developed under zero pressure gradient conditions. The vortex street was generated by a cylinder positioned in the free stream. An X-type hot-wire probe located in the boundary layer, measured the streamwise and normal to the wall velocity components. The measurements covered two areas; the region of transition onset and development and the region where the wake and the boundary layer merged producing a turbulent flow. The evolution of Reynolds stresses and rms-values of velocity fluctuations along the transition region are presented and discussed. From the profiles of the Reynolds stress and the mean velocity profile, a `negative' energy production region along the transition region, was identified. A quadrant splitting analysis was applied to the instantaneous Reynolds stress signals. The contributions of the elementary coherent structures to the total Reynolds stress were evaluated, for several x-positions of the near wall region. Distinct regions in the streamwise and normal to the wall directions were identified during the transition.

Velocity statistics along the stagnation line of an axi- symmetric stagnating turbulent flow

Velocity statistics along the stagnation line of an axi-symmetric wall stagnating turbulent flow are studied experimentally. A low turbulence, uniform air flow from a nozzle type air supply with an exit diameter of 50 mm stagnates at a wall located 50 mm downstream. A flow velocity is set to 3 m/s, 10 mm downstream from the exit of the air supply. Instantaneous values of streamwise and radial velocities are measured by laser-Doppler velocimetry. The turbulence level in the air flow is changed by use of turbulence generator. When the turbulence generator is not installed in the air supply, the mean velocity profile in the streamwise direction fits well with that of a laminar viscous flow with the rms value of velocity fluctuations low near the wall. With the turbulence generator installed, a significant turbulence structure appears near the wall. When the wall is approached, the rms value of velocity fluctuations in the streamwise direction decreases monotonically while the profile of the rms value in the radial direction reaches a maximum near the wall. The increase in the rms value of velocity fluctuations in the radial direction near the wall is attributed to the bi-modal histogram of the fluctuating velocity in the radial direction. Near the wall, the instantaneous stagnation streamline fluctuates and the probability of the mean location of the stagnation point reaches a maximum not at the stagnation line but on a circle around the stagnation line, resulting in the bi-modal histogram. Turbulence statistics, the rms value of velocity fluctuation and the turbulent kinetic energy, can be normalized successfully by similarity parameters based on the strain rate and the reference turbulent kinetic energy introduced by Champion and Libby.

Long Time Scale Fluctuation in the Irregularity of Vortex Shedding. Spectral Analysis of the Local rms Value and Criculation.

Irregularity of vortex shedding from a circular cylinder is studied experimentally. The study is based on a spectral analysis of local rms value of velocity fluctuation behind the circular cylinder. Spectral analysis of circulation of vortices obtained by both PIV and image processing of the streak line visualized by hydrogen bubbles extending from the location near the separation point of shear layer is made in order to investigate the relationship between velocity fluctuations and circulation fluctuations. From this study, long time scale periodic fluctuation, approximately 20 times the vortex shedding periodicity, is clarified. Also a good correlation between the time variation of the local rms value and that of the circulation was obtained.

Laser velocimetry measurements in a horizontal gas-solid pipe flow

This paper presents laser measurements of particle velocities in a horizontal turbulent two-phase pipe flow. A phase Doppler particle analyzer, (PDPA), was used to obtain particle size, velocity, and rms values of velocity fluctuations. The particulate phase consisted of glass spheres 50 μm in diameter with the volume fraction of the suspension in the range φp=10-4 to φp=10-3. The results show that the turbulence increases with particle loading.