“Analysis of the propeller wake evolution by pressure and velocity phase measurements”

In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller angle, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to two diameters downstream. In phase pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model at different advance ratios. Pressure data, processed by using slotting techniques, allowed reconstructing the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream of the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency.     

Phase sampling in the analysis of a propeller wake

A phase sampling procedure is used for the analysis of the non-steady, periodic flow field in the near wake of a marine propeller. This method allows to obtain a true ensemble averaging of the experimental measurements. The average is made over a large number of repeated experiments each of which is taken during a complete revolution of the propeller. The measurements are carried out in a recirculating water tunnel with a two-channel laser Doppler velocimeter. The computer-aided evaluation of the experimental results visualizes the following characteristic features of the wake: (1) the vortex sheet developing from the trailing edge; (2) a sudden increase of the axial velocity in the core of the tip vortex; (3) a boundary layer effect near the shaft of the propeller. From the analysis of the direction of vortex rotation along the radial direction of the blade, it is possible to derive information on the working conditions of the propeller.     

Analysis of unstable vortical structure in a propeller wake affected by a simulated hull wake

A two-frame PIV (particle image velocimetry) technique was used to investigate the flow characteristics of a complicated propeller wake influenced by a hull wake. As the propeller is significantly affected by the hull wake of a marine vessel, measurements of the propeller wake under the hull wake are certainly needed for more reliable validation of numerical predictions. Velocity field measurements were conducted in a cavitation tunnel with a simulated hull wake. Generally, the hull wake generated by the hull of a marine ship may cause different loading distributions on the propeller blade in both the upper and the lower propeller planes. The unstable propeller wake caused by the ship’s hull was interpreted in terms of turbulent kinetic energy (T _KE) to obtain useful information for flow modeling. The unstable or unsteady phenomenon in the upper propeller wake was identified by using the proper orthogonal decomposition (POD) method to characterize the coherent flow structure with turbulent kinetic energy. Strong unsteadiness appeared in the second and higher modes, largely affecting the downstream flow characteristics. The first eigenmode can be used to appropriately identify the tip vortex positions even in the unstable downstream region, which are helpful for establishing reliable wake modeling.     

The ultra-low Reynolds number airfoil wake

Lift force and the near wake of an NACA 0012 airfoil were measured over the angle (α) of attack of 0°–90° and the chord Reynolds number (Re _c ), 5.3 × 10³–5.1 × 10⁴, with a view to understand thoroughly the near wake of the airfoil at low- to ultra-low Re _c . While the lift force is measured using a load cell, the detailed flow structure is captured using laser-Doppler anemometry, particle image velocimetry, and laser-induced fluorescence flow visualization. It has been found that the stall of an airfoil, characterized by a drop in the lift force, occurs at Re _c  ≥ 1.05 × 10⁴ but is absent at Re _c  = 5.3 × 10³. The observation is connected to the presence of the separation bubble at high Re _c but absence of the bubble at ultra-low Re _c , as evidenced in our wake measurements. The near-wake characteristics are examined and discussed in detail, including the vortex formation length, wake width, spanwise vorticity, wake bubble size, wavelength of K–H vortices, Strouhal numbers, and their dependence on α and Re _c .     

Three-component velocity field measurements of propeller wake using a stereoscopic PIV technique

A stereoscopic PIV (Particle Image Velocimetry) technique was used to measure the three-dimensional flow structure of the turbulent wake behind a marine propeller with five blades. The out-of-plane velocity component was determined using two CCD cameras with an angular displacement configuration. Four hundred instantaneous velocity fields were measured for each of four different blade phases, and ensemble averaged in order to find the spatial evolution of the propeller wake in the region from the trailing edge up to one propeller diameter (D) downstream. The influence of propeller loading conditions on the wake structure was also investigated by measuring the velocity fields at three advance ratios (J=0.59, 0.72 and 0.88). The phase-averaged velocity fields revealed that a viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were generated periodically and the slipstream contracted in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. As the flow moved downstream, the turbulence intensity, the strength of the tip vortices, and the magnitude of the out-of-plane velocity component at trailing vortices all decreased due to effects such as viscous dissipation, turbulence diffusion, and blade-to-blade interaction.     

Dependence of flow classification on the Reynolds number for a two-cylinder wake

This work aims to investigate the dependence of flow classification on the Reynolds number (Re) for the wake of two staggered cylinders. The Re examined ranges from 1.5×10³ to 2.0×10⁴. The pitch ratio, P^⁎=P/d examined is 1.2–6.0 (d is the cylinder diameter), and angle (α) is 0–90°, where P is the center-to-center spacing between two cylinders and α is the angle between the incident flow and the line through the cylinder centers. Two single hotwires were used to measure simultaneously the fluctuating streamwise velocities (u) in the vortex streets generated by the two cylinders. The power spectral density functions and the Strouhal numbers were then obtained from the u signals, based on which the flow structure pattern or mode could be determined. Over two hundred configurations of two staggered cylinders have been examined for each Re. It is found that Re has an appreciable effect on the dependence of the flow mode on P^⁎ and α. The observation is connected to the Re effect on the generic features of a two-cylinder wake such as flow separation, boundary layer thickness, gap flow deflection and vortex formation length.     

Finding ship propeller profiles with the least critical cavitation number

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 47–52, September–October, 1991.     

Streamwise evolution of an inclined cylinder wake

The streamwise evolution of an inclined circular cylinder wake was investigated by measuring all three velocity and vorticity components using an eight-hotwire vorticity probe in a wind tunnel at a Reynolds number Re_d of 7,200 based on free stream velocity (U _∞) and cylinder diameter (d). The measurements were conducted at four different inclination angles (α), namely 0°, 15°, 30°, and 45° and at three downstream locations, i.e., x/d = 10, 20, and 40 from the cylinder. At x/d = 10, the effects of α on the three coherent vorticity components are negligibly small for α ≤ 15°. When α increases further to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of the streamwise vorticity increases by about 70%. Similar results are found at x/d = 20, indicating the impaired spanwise vortices and the enhancement of the three-dimensionality of the wake with increasing α. The streamwise decay rate of the coherent spanwise vorticity is smaller for a larger α. This is because the streamwise spacing between the spanwise vortices is bigger for a larger α, resulting in a weak interaction between the vortices and hence slower decaying rate in the streamwise direction. For all tested α, the coherent contribution to [`(v<sup>2</sup>)] \overline{{v^{2}}} is remarkable at x/d = 10 and 20 and significantly larger than that to [`(u²)] \overline{{u^{2}}} and [`(w<sup>2</sup>)]. \overline{{w^{2}}}. This contribution to all three Reynolds normal stresses becomes negligibly small at x/d = 40. The coherent contribution to [`(u²)] \overline{{u^{2}}} and [`(v²)] \overline{{v^{2}}} decays slower as moving downstream for a larger α, consistent with the slow decay of the coherent spanwise vorticity for a larger α.     

Approximate calculation of three-dimensional cavitating flow around propeller blades in a viscous capillary fluid

Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 83–90, November–December, 1990.     

Numerical investigation of the effect of winglets on the evolution of an aircraft vortex wake

The effect of winglets on the aerodynamic characteristics of a heavy aircraft and the parameters of the vortex wake behind it is investigated for the landing regime. The solution of the complete problem is obtained by breaking up the wake into three regions, namely, the near, intermediate, and far flowfields, in which the corresponding subproblems are successively solved. The wake flowfields are obtained at different distances from the aircraft, together with the distributions of the mean azimuthal velocity over the vortex radius, and the lifetime of the vortex system is estimated for several one-and two-element wingtip versions. All the results are compared with those for the baseline layout with no winglets.     

Effect of the wake on flow in an annular jet

The flow of an annular jet in a channel is studied. The effect of the Mach number of the wake on the structure of the jet is determined. It is shown that with a near-sonic velocity a reorganization occurs in the mode of flow from an open to a closed base region accompanied by a reduction in the level of pressure pulsations.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 74–79, November–December, 1974.     

TR-PIV measurement of the wake behind a grooved cylinder at low Reynolds number

A comparative study of the wakes behind cylinders with grooved and smooth surfaces was performed with a view to understand the wake characteristics associated with the adult Saguaro cacti. A low-speed recirculation water channel was established for the experiment; the Reynolds number, based on the free-stream velocity and cylinder diameter (D), was kept at Re_D=1500. State-of-the-art time-resolved particle image velocimetry (TR-PIV) was employed to measure a total of 20 480 realizations of the wake field at a frame rate of 250 Hz, enabling a comprehensive view of the time- and phase-averaged wake pattern. In comparison to the wake behind the smooth cylinder, the length of the recirculation zone behind the grooved cylinder was extended by nearly 18.2%, yet the longitudinal velocity fluctuation intensity was considerably weakened. A global view of the peaked spectrum of the longitudinal velocity component revealed that the intermediate region for the grooved cylinder, which approximately corresponds to the transition region where the shear layer vortices interact, merge and shed before the formation of the Karman-like vortex street, was much wider than that for the smooth one. The unsteady events near St=0.3-0.4 were detected in the intermediate region behind the grooved cylinder, but no such events were found in the smooth cylinder system. Although the formation of the Karman-like vortex street was delayed by about 0.6D downstream for the grooved cylinder, no prominent difference in the vortex street region was found in the far wake for both cylinders. The Proper Orthogonal Decomposition (POD) method was used extensively to decompose the vector and swirling strength fields, which gave a close-up view of the vortices in the near wake. The first two POD modes of the swirling strength clarified the spatio-temporal characteristics of the shear layer vortices behind the grooved cylinder. The small-scale vortices superimposed on the shear layers behind the grooved cylinder were found to be generated and convected downstream in the same phase, which would significantly reduce the fluctuating force on the cylinder surface.     

Experimental investigation of the effect of Reynolds number on flow structures in the wake of a circular parachute canopy

In the present study, an experimental study was conducted to characterize the effect of Reynolds number on flow structures in the turbulent wake of a circular parachute canopy by utilizing stereoscopic particle image velocime- try （Stereo-PIV） technique. The parachute model tested in the present study was attached by 28 nylon suspension lines and placed horizontally at the test section center of the wind tunnel. The obtained results showed that with the in- crease of Reynolds number, the intensities of the vortices near the downstream region of the canopy skirt were found to increase accordingly. However, the increase of Reynolds number did not result in a significant change in ensemble- averaged normalized x-component of the velocity, ensembleaveraged normalized vorticity, normalized Reynolds stress, and normalized turbulent kinetic energy distributions in the turbulent wake of the circular parachute canopy. The obtained results are very useful to further our understanding about the unsteady aerodynamics in the wake of flexible circular parachute canopies and to constitute a reference for CFD computation.     

Investigation of the three-dimensional intermediate wake topology for a square cylinder at high Reynolds number

Simultaneous multi-point hotwire measurements are used to investigate the three-dimensional wake topology of a square cylinder at high Reynolds numbers. Wavelet techniques are applied to detect the flow structures and to inquire on the validity or extension of previously proposed low Reynolds number topological models to turbulent wakes. Our results suggest that a flow topological model similar to the horizontal perturbation model proposed by Meiburg and Lasheras (J Fluid Mech 190:1–37, 1988) but with alternate rib cuts in the horizontal plane is plausible for the intermediate wake topology.

---

Turbulization of the wake behind a single roughness element on a blunted body at a hypersonic Mach number

The influence of a cylinder-shaped single roughness element on the laminar–turbulent transition in the presence of an entropy layer is experimentally studied. The experiments are performed on a blunted cone model at the Mach number M = 5. The roughness element is located on the blunted tip of the model. Information about the mean and fluctuating parameters of the boundary layer in the wake behind the roughness element is obtained by using hot-wire anemometry. It is shown that flow turbulization behind the roughness elements occurs at the local Reynolds number calculated on the basis of the roughness element height and equal to 400–500. It is found that the presence of the roughness element exerts a significant effect on the unsteady characteristics of the boundary layer if the roughness element height is smaller than the effective value.     

Experimental study of the evolution of perturbations in the supersonic wake behind a flat plate

The evolution of perturbations in the wake of a flat plate (with symmetric wedge-shaped nose and tail) is studied experimentally at the Mach number M=2 for various plate thicknesses and tail lengths.Novosibirsk. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 167–171, July–August, 1996.     

Effects of aspect ratio and orientation on the wake characteristics of low Reynolds number flow over a triangular prism

The wake characteristics of unconfined flows over triangular prisms of different aspect ratios have been numerically analysed in the present work. For this purpose, a fixed Cartesian-grid based numerical technique that involves the porous medium approach to mimic the effect of solid blockage has been utilised. Correspondingly, laminar flow simulations ranging from the sub-critical regime (before the onset of vortex shedding) to the super-critical regime have been considered here within the limits of two-dimensionality. In the sub-critical regime, correlations relating the wake bubble length with Reynolds number (Re) have been proposed for various aspect ratios. Also, the effects of aspect ratio and Reynolds Number on the drag force coefficient (C_D) have been characterised for two different geometrical orientations of the prism (base or apex facing the flow). Subsequently, the critical Reynolds number at the onset of vortex shedding has been predicted for each of the aspect ratio considered, by an extrapolation procedure. The unsteady flow characteristics of the super-critical regime are finally highlighted for different aspect ratios and triangular orientations considered in the study.     

Effect of mini-flaps on the structure of the near wake behind a model half-wing

The effect of mini-flaps on the vortex structure of the near wake flow behind a model of a half-wing, rectangular in plan, is investigated. In a subsonic wind tunnel the time-average flow parameters are measured in several sections behind a model with flaps mounted on both upper and lower surfaces near the trailing edge. The wake flow parameters are compared with those for a model with no flaps. The considerable effect of the flaps on the flow structure in the viscous core of a tip vortex formed behind the model half-wing is established.