Eighteenth Symposium on Naval Hydrodynamics (1991) / Chapter Skim
Currently Skimming:
Numerical and Experimental Anlysis of Propeller Wake by Using a Surface Panel Method and a 3-Component LDV
Numerical and Experimental Anlysis of Propeller Wake by Using a Surface Panel Method and a 3-Component LDV
Pages 297-318
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 297... ...
S Sj t1 ~t2 T V, V] vt Normal coordinate f or blade section Propeller rotational speed, [ rps ~ Unit vector outward normal to surface Total number of blade and hub panels Number of chordwise blade panels Number of radial blade panels Propeller center Pressure Static pressure at inf inity Pitch of blade section Pitch of trailing vortex sheet Propeller torque Radial coordinate f rom propeller axis = ~y2+z2 Propeller radius = D/2 Radius of propeller hub Radius of hub vortex Radius of ultimate wake Distance between field point P and boundary point Q Distance between the i-th control point and the j-th integration point Chordwise coordinate f or blade section Chordwise coordinate of leading edge Boundary surf ace Surface of the j-th panel Tangential coordinates on panel Propeller thrust Cartesian coordinates in the blade-f ixed f rame XR (r)
|
From page 298... ...
Based on the measured velocity distributions of the propeller slipstream, a deformed wake model is proposed where the contraction of the slipstream and the variation of the pitch of the inboard helical trailing vortex sheets are taken into account. Then, pressure distributions on the propeller blade and open-water characteristics calculated by the present method are compared with the experimental data.
|
From page 299... ...
We consider a boundary surface S which is composed of propeller blade surface SB, hub surface SH and wake surface Sw, and also unit outward normal vector n to the surface S
|
From page 300... ...
In the past application of the panel method to the propeller problem, planar quadrilateral panel has been used to approximate the surface. However, the elements representing the propeller blades must be nonplanar due to the helical blade surface, which results in gaps at the edge of the planar panel and therefore numerical errors.
|
From page 301... ...
A detailed formulation of the numerical Kutta condition is shown in APPENDIX. 3.3 Velocity and Pressure on the Surface The velocity and pressure distributions on the blade and hub surfaces can be evaluated directly by taking the gradient of the influence coefficients for the velocity potential Eq.~13~.
|
From page 302... ...
3.5 Thrust and Torque of Propeller The total forces and moments acting on a propeller can be obtained by integrating the pressures over the blade and hub surfaces. Denoting the components of the outward normal vector ni by (nxi' nyi, nzi)
|
From page 303... ...
If the propeller rotates with a constant angular velocity, the time dependent velocity measurements at a certain radius correspond to the measurements at the same radius for many different angular positions of the measuring points at a fixed propeller position. Hence, the velocity measurements with respect to a certain propeller position give the instantaneous velocity distribution of the propeller at a certain time.
|
From page 304... ...
Fig.7 shows the velocity fluctuations at axial position of x/rO=0.25 just behind the propeller. The sudden change of the radial velocity component shows the velocity jump across the trailing vortex sheet.
|
From page 305... ...
This means that the pitch of the tip vortex is smaller than that of the trailing vortex sheet. 4.5 Hydrodynamic Pitch Using the circumferentially averaged axial and tangential velocities v-x, Van, hydrodynamic pitch angle of the trailing vortex sheets in the propeller slipstream can be calculated by v = tan~1 ( Rev )
|
From page 306... ...
. Fig.9-a Velocity distributions downstream of propeller B ~ J=0.70 )
|
From page 307... ...
Pitch of the tip vortices is obtained from the axial variations of the angular positions of the center of the tip vortices and also plotted in Figs.ll - 13. Pitch of the tip vortices is considerably smaller than that of the inboard helical trailing vortex sheets.
|
From page 308... ...
~ fit Transition Wake Ultimate Wake 1 -' ran ran , ~ - t Fig.15 Model of propeller slipstream inboard helical trailing vortex sheets and the decrease in pitch of the tip vortices near outer edge of the slipstream. 4.6 Numerical Modeling of Trailing Vortex Sheet A linear wake model of the trailing vortex sheets which was based on the geometrical pitch of blades and ignored the contraction of slipstream was used in the previous papert203.
|
From page 309... ...
The deformed wake model based on the measurements of the flow field around the propeller are compared with the conventional linear wake model in Fig.17. Large deformation of the tip vortices can be observed in the new wake model, which is similar to the wake model based on the measured wake pitch shown by Jessup [27~.
|
From page 310... ...
The f irst solution which corresponds to the application of the Morino Kutta condition gives large discrepancy of the pressure on the upper and lower sides at the trailing edge, while the second and third solutions give almost equal pressure at the trailing edge by the application of the iterative Kutta condition. It can be pointed out that the convergence of the present iterative Kutta condition is very fast.
|
From page 311... ...
-0.1 _ Suction Side r/ro =0.7 ;~-: 1 ~=: /' Pressure Side 'I 0.2 I Suction Side r/ro = 0.9 1~1 r ~ Pressure Side -01 0.0 0.2 0.4 0.6 0.8 ~1.0 Fraction of Chord Fig.22 Comparison of chordwise pressure distributions with and without a hub for DTRC P4718 5~2 Field Point Velocity In order to evaluate the accuracy of the present panel method, flow fields around propeller were calculated for both the linear wake and the deformed wake models and compared with the measurements by LDV. Each of the three propeller models A, B and C is replaced with 240 panels per blade (NR=12, NC=10)
|
From page 312... ...
Measured by LDV In' " x/rO = 0.50 Calculation with Deformed Wake Calculation with Linear Wake z z I'. it, _y Fig.25 Comparison of cross components of velocities downstream of propeller B ( J=0.70, x/r =0.50 )
|
From page 313... ...
There is, however, disagreements in the axial velocities at outer radii for the calculations with the linear wake model. It can be said that close agreement between the calculations and the measurements is due to the consideration of both the contraction of the propeller slipstream and the variation of pitch of the trailing vortex sheets in the present calculation.
|
From page 314... ...
Further, f low f ields around three propeller models were investigated precisely by using a 3-component LDV. Based on the measured velocity distributions of the slipstream, a def armed wake model of the helical trailing vortex sheets behind propeller was proposed.
|
From page 315... ...
D ., "Further Measurements of Model Propeller Pressure Distributions Using a Novel Technique, " DTNSRDC-86/011, May 1986, David Taylor Research Center, Bethesda, Md.
|
From page 316... ...
However, variations of the radial positions and the pitch distributions of the trailing vortices in the transition wake region would be too difficult to be estimated by numerical calculations only. Therefore, a deformed wake model based on LDV-measurement was proposed in the present paper.
|
From page 317... ...
Precise measurement of flow fields around a hub vortex would be indispensable to construct a more realistic hub vortex model. DISCUSSION Shi-Tang Dong China Ship Scientific Research Center, China Congratulate on author's excellent work.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.