Detailed propeller design often relies on a description of the inflow wake distribution in the plane of the propeller in order to properly design the blade shapes to maximize efficiency, minimize cavitation, and reduce the effects of the unsteady propeller forces (like noise and vibrations). For many years scale model testing has been used to predict the wake distribution by running tests either in a towing tank or a flow channel over a range of vessel speeds and using a measurement device (such as pitot tubes or Laser Doppler Velocimetry ) to measure the local flow velocities in the plane of the propeller. It is possible to use Orca3D Marine CFD to simulate and measure the "nominal" wake distribution. This approach has the benefit of not being limited to Reynolds numbers associated with smaller scale models since the CFD simulations can be run at full scale.
Beginning with Orca3D Version 3, the setup and reporting for a propeller wake survey is fully automated. When running the OrcaSimericsAnalysis command that is part of Orca3D Marine CFD, we've added the option for a "Wake Survey" analysis type as shown below:
Once you select this analysis type, the Propulsors input tab will change to allow you to specify both the location of the wake disk as well as the distribution of measurement points in the wake survey as shown below:
The SimericsMP simulation will then run as a standard resistance (towed) analysis but will also add the necessary wake measurement points and expressions to plot and evaluate the nominal wake. Once you have completed the simulation, there is a new option in the Orca3D Marine CFD reporting command (OrcaCreateCfdReport) that automatically generates the wake survey report. The reporting command input form is shown below, and below that is a typical report output from the command.
In order to simulate a propeller wake survey with Orca3D Marine CFD with Orca3D Version 2, the process involved using a semi-automated approach. The following steps should be performed:
1. First set up one or more CFD powering simulations in Orca3D Marine CFD, and use the "Create Run Files Only" option to create the files. This requires entering the information for the propeller(s).
2. Wake measurements are traditionally made in resistance simulations with appendages but in the absence of the propeller, so in Simerics you will switch back to a Prescribed Profile to define the speed, rather than Self-Propelled. In Simerics, open the project file (.spro) that you created in step 1. Select the "Marine" module in the Model panel, an change the Propulsion Option from Self-Propelled to Prescribed Profile. Enter the Target Velocity (note that it is in m/s by default).
3. The wake survey process relies on distributing a series of "monitoring points" in SimericsMP over the plane of the propeller. As the name implies monitoring points allow the user to monitor the local flow behavior, including flow velocities, at specific locations. The locations of the monitoring points are automatically computed for you. Once you open the simulation in SimericsMP and change to Prescribed Profile, there is an option to set up a wake survey by adding a special Propulsion Source. As shown in the image below first select the "Marine" module in the Model panel and then in the Properties panel expand the Propulsion Option. Expand the first Propulsion Source (this is the propeller that you defined in Orca3D) so that you can see the values. Now click on the dropdown for the second Propulsion Option and choose "Add Wake Survey" to add a new Wake Survey. This type of propulsion source is a special type in that it does not add any propulsive force to the simulation but instead allows some of the same inputs that are used to define a propeller to be specified along with others to define the disk within which the wake survey is desired.
Once you've added the wake survey source, expanding it reveals the required input parameters as shown below. Define the Wake Survey by entering a Propulsion ID (an integer, for example 2), and copying the Position, Direction, and Hub Diameter from the first Propulsion Source. The "Wake" diameter is generally the same as the propeller diameter. Once this is complete, go back to the first Propulsion Source and select Remove.
Two additional inputs include the number of monitoring points to create within the disk both radially and circumferentially.
After that is done you can select the "Preview Monitor Points" dropdown and select Yes to create the points. Alternatively they will be automatically created when you run the simulation.
This is all that is required to define the locations of the monitoring points. However, one additional feature of the semi-automated approach described here is that SimericsMP will automatically perform an integration of the longitudinal flow velocities at each point to determine an average or "nominal" wake fraction. To plot the computed wake fraction you will need to define a custom plot variable as shown below. Note that the integer value "2" in that expression must match the ID of the Wake Survey. This allows for multiple wake survey sources to be added if needed.
After defining the custom plot variable and running the simulation you can plot the wakeFraction value as shown below. In this context the variable "wakeFraction" refers to the wake fraction coefficient, "w".
If you would like to visualize the distribution of the wake fraction over the propeller plane, you will need to first define a custom display variable in SimericsMP and then create a section at the plane of the propeller on which you choose to display the custom variable. Creating custom display variables is discussed in the article, Creating Custom Display Variables. In this example, define a display variable called display.wakeFraction in the expression editor as shown below. The numerator in that expression, (marine.forward_vel + flow.V.x) is sensitive to the forward direction of the SimericsMP model. In this case +X is in the aftward direction, but if +X is the forward direction then the numerator would be (marine.forward_vel - flow.V.x).
Here the wakeFraction, used in this context to represent "1-w", is computed as the sum of the forward velocity and the x component of the local flow velocity (in inertial coordinates) non-dimensionalized by the forward velocity. Defining an X = constant section at the plane of the propeller and plotting the custom variable gives the plot shown below:
Note the yellow and orange areas near the propeller hub represent areas of flow greater than free stream velocity, with the light green and blue regions represent a wake deficit. The large vertical blue region at the right of the image above is a flow deficit behind the centerline skeg on this vessel.