ESLOFT User Guide DFDC v070-ES2a ------------------------------------------------------------------------------ Esotec Developments, April 2009 philip (at) esotec (dot) org Last update: 28 Oct 2009 BACKGROUND ESLOFT is a tool for getting rotor and stator blade designs out of DFDC and into CAD for subsequent 3D modeling, meshing or manufacture. This is accomplished by calculating on the rotor/stator geometry along with blade thickness data (input) and airfoil geometries (parent airfoils) to generate points files which can be lofted in 3D CAD systems. Specification of blade thickness plays a central role in ESLOFT. A smooth thickness distribution is desired for both aerodynamic and structural reasons. Since chord is defined by the blade design, defining the distribution of thickness (t) or thickness/chord (t/c) along the blade allows for section t/c to be determined at any station radius. Sections are interpolated to the required t/c from parent airfoils bounding that t/c (or extrapolated as necessary). A uniform paneling scheme is used for the interpolation of parent airfoils. Each parent is paneled with the specified points per side and spacing factor (space_te/space_le) relative to the airfoil arc length (te-le), hence permitting simple linear interpolation between parent airfoil coordinate points relative to t/c. Note: To gain insight into this paneling/interpolation scheme, execute BLEN 0 then BLOW to zoom in close to the leading edges. Compatible parent airfoils will generally produce children that perform every bit as well as their parents. Interpolating between incompatible parents might yield a section better than them both - or worse, as the case may be. Extrapolation works but should be handled with care. GENERAL PROGRAM EXECUTION At DFDC top level: LOAD < case file > LOFT You can work with parent airfoils in ESLOFT without a loaded case file, but can do little else. The case file must include at least one bladed rotor or stator. ESLOFT runs the general geometry setup routine GENGEOM when starting up (as when entering OPER - although only blade paneling is needed the routine is used for simplicity's sake). When first entering ESLOFT the user must load a parent airfoil set (LSET) or at least two individual parent airfoils (LDAT) before proceeding further. The program operates on four execution levels: 1. Parent airfoil processing. 2. Loft configuration and thickness distribution. 3. Blended (interpolated, normalized) sections. 4. Transformed (dimensioned and rotated) sections. Calculations are executed only when required, transparently to the user. When a case file (with a bladed rotor or stator) and at least two parent airfoils are loaded, all commands are available. The loft is recalculated and displayed whenever loft parameters are modified, or it can be displayed at any time with SHOW (or s). ESLOFT is seamlessly integrated with DFDC. Switch between disks (rotors/stators) and the loft will reflect the new geometry. If you leave ESLOFT and return without quitting DFDC, parent airfoils and loft settings will remain as you left them and the loft will reflect any subsequent changes made to rotor/stator geometry. If you have sufficient confidence in the default settings, the minimum command sequence for calculating a loft in ESLOFT and saving dimensioned points files to disk is as follows: LSET < .esloft airfoil file > SAVT 0 < cr > to select default filenames Done. PARENT AIRFOILS LSET f Load parent airfoil set from disk SSET f Save parent airfoil set to disk DELP i Delete parent airfoil from set LDAT f Import an airfoil from disk SDAT i Save an airfoil to disk ALFZ i Specify parent airfoil zero-lift alpha PANE ir Specify pnts/side & spacing, repanel parents NAMP s Specify parent airfoil set name DISP(d) Display parent airfoil set To streamline parent airfoil management ESLOFT uses its own file format for parent airfoil sets. ESLOFT files contain paneled coordinates for (up to 10) parent airfoils along with their names and zero-lift alphas. Geometry data for each airfoil is calculated when a set is loaded. Assemble your own airfoil sets by loading airfoils (LDAT) and saving (SSET). Or use one of the example sets. When an individual airfoil is loaded (LDAT) it is processed as follows: - the section is normalized to unit chord. - the section is repaneled to current parameters. - geometry data is calculated and stored. - the user is asked to input the section's zero-lift alpha (A0). ESLOFT parent airfoil sets are always ordered according to decreasing t/c. Whenever an airfoil is added to or deleted from a set the arrays are shuffled to maintain this ordering. A parent set cannot contain two airfoils of the same t/c. If an airfoil with t/c within 0.005 (0.5%) of a current parent is loaded, the user has the option of overwriting or aborting. This is a somewhat arbitrary limitation but a sensible one. Since sections are interpolated relative to t/c, dissimilar parents of similar t/c will produce short period waves in the blade surface. For best results, use only as many parents as necessary and adjust the thickness distribution to achieve appropriate spacing between parents on the t/c curve. A parent airfoil set must contain at least two parents. If you need more than 10 parents increase NAFX. All sections in an .esloft file are normalized and indentically paneled - don't try to create an .esloft file manually. Repaneling (PANE) always applies to the whole parent set. Hence, all sections in ESLOFT's arrays (parent, blended or transformed) are paneled to the same parameters. Current max points per side is 100 - increase NPX for more. DISP (or d) displays parent airfoil data and plots the set. For more detailed parent airfoil plots, use PARE. Use SDAT to save a parent airfoil to a standard XFOIL .dat file for analysis or modification. If t/c remains similar the user can reimport the modified airfoil (LDAT) and overwrite the current version directly. A Note on Alpha_Zero ESLOFT interpolates input A0 data to the blended sections relative to t/c and makes a correction to beta upon comparison with A0 data used by AERO. The reasoning is as follows. A0 is the dominant airfoil-related parameter affecting beta while varying widely between sections (driven mainly by camber). When setting up parameters in AERO it will not always be clear where a particular section will be located on the blade. Hence it is convenient to keep this data with the airfoil geometry itself and make the correction in ESLOFT. Hence, A0 settings in AERO do not influence the lofted beta angles. It remains important to set up the AERO parameters carefully, however, in particular dCL/dAlpha, the lift curve slope, which also has a direct influence on beta, though the variance between airfoil sections will typically be less than that of A0. LOFT CONFIGURATION STN ir Specify loft station number and spacing TGAP r Specify rotor geometric tip gap OSHO rr Specify overshoot at hub and tip PAX rr Specify pitch axes at hub and tip (x/c) DIH r Specify dihedral at tip (circular arc) THUB r Specify blade thickness at hub TTIP r Specify blade thickness at tip TYPE i Specify thickness distribution type PARA r Specify parabolic axis SHOW(s) Display current loft configuration WRIT f Save current loft data to disk DEF Reset loft parameters to default settings BANG Toggle local/global beta coords (BGAM<0) DIM Toggle 2D/3D lofted points files ROTA Toggle left/right hand rotation UNIT i Specify units for loft output NAML s Specify base name for loft output SHOW (or s) calculates (if necessary), displays and plots the current loft configuration. When first calculated the loft will be configured to default settings, as follows: Stations 16 Station spacing 2.0 (space_tip/space_hub) Tip Gap 0 or as set in OPER Overshoot 0 at hub and tip Pitch axes 0.3 at hub and tip (x/c) Dihedral 0.0 (mm offset from camberline) Hub thickness 0 - set by thickest parent located at hub Tip thickness 0 - set by thinnest parent located at tip Thickness Distrn Linear t/c (hub-tip) Parabolic axis 0.5 (for parabolic t and t/c) Execute DEF at any time to return the loft to these settings. The default settings are set up to be 'safe' - they will produce a reasonable loft in the majority of rotor cases (assuming reasonable parents) and provide a platform for subsequent adjustments. Note that DEF resets loft configuration settings only, not parent airfoils or output settings. Current max stations is 30 (with 2 reserved for overshoot). Increase NLSX if you need more. TGAP is the same command as in OPER, but accepts mm units and ignores the secondary input. Overshoot (OSHO, mm) extends the blade beyond nominal length - to facilitate fitting to the hub, for instance, or for manufacturing purposes. Overshoot adds just one station beyond the hub and/or tip (with splined chord and beta) and has no effect on nominal blade geometry. DIH curves the blade in a circular arc root to tip, in the direction of the rotational axis, specified in mm at the tip. This may be used to offset centrifugal forces with thrust forces, leading to lower stresses in the blade. Specifying Blade Thickness Creating a suitable thickness distribution for a given rotor/stator and parent airfoil set is the heart of the lofting process. This is done in two steps: 1. Specify thickness at hub and tip. 2. Specify the thickness (or t/c) distribution. Hub and tip thickness can be specified implicity (by locating a parent airfoil there) or explicitly (THUB and TTIP - mm). Set THUB or TTIP to 0 to set hub or tip thickness implicitly by the thickest (or thinnest) parent t/c. To locate another parent at the hub or tip, simply delete the unneeded parent (DELP i). When entering hub or tip thickness explicitly, care is needed to avoid unreasonable extrapolations. Add parents of suitable t/c or delete airfoils from the parent set as required. Thickness Distributions Four thickness distribution types are available: 1 Linear t/c 2 Linear t 3 Parabolic t/c 4 Parabolic t Execute TYPE i to select these directly. Specifying t/c tends to work best for rotors, while specifying t can work better for stators whose chords are generally more constant. The parabolic distributions fit a parabolic curve to the hub and tip t (or t/c) with the user controlling the location of the parabolic 'x' axis (in units of bladelengths inboard of the hub). Use PARA to move the axis. A smaller value will increase curvature and vice versa. Obviously the axis cannot be moved outboard of the inboard station. Note: Parent airfoil radial locations The loft configuration locates each parent at the blade radius where its t/c intersects the t/c curve. When specifying t (either linear or parabolic) the parent is located with the help of an inverse-spline routine. When specifying t it is also possible to generate a t/c curve with an intermediate maxima (between hub and tip), in which case there will be either two solutions or none. The inverse-spline routine begins its hunt mid-blade, so a maxima inboard of midblade will see parents located at the outboard solution. A parent with a t/c larger than the maxima could end up anywhere. Error messages are displayed if the routine blows up. All of this is of little concern, however. Parent airfoil radial locations are displayed to help the user appropriately control t or t/c, but serve no further purpose. All subsequent interpolations (of geometry and data) are done relative to t/c and will calculate normally. Nevertheless, keep in mind that parent-location errors generally indicate that the loft needs adjustment. Secondary Loft Settings BANG (local/global coordinates for neg BGAM disks) works the same as in OPER. It is applied to loft configuration data and has no effect on transformed section output. The DIM toggle allows for 2D or 3D points-file output, with 3D adding the third Z (radial) dimension. Use ROTA to toggle between left and right hand rotation. The default is left hand (leading edge to the left). Note that this setting applies directly to positive-rpm rotors. Negative BGAM disks (stators and neg-rpm rotors) will always be output with the reverse orientation. This applies to transformed section plots and points-file output. Output UNITs can be meters, centimeters, millimeters or inches. This setting is reflected in transformed section plots and points-file output. NAML allows the specification of a loft output base name different from the case name (the default), anticipating multiple lofts of the same case. When writing multiple points files to disk the user has the option of manually naming each file or having ESLOFT create unique filenames by appending suffixes to the base name (recommended). In the case of overwriting files, the user is given the options of aborting or overwriting just one file or all. Unique filenames are assembled as follows: Blended sections : base_name -dsk# -stn# .dat Transformed sections: base_name -dsk# -stn# .txt Station radius file : base_name -dsk# -radii.txt Disk numbers are omitted for single-disk cases. SECTION PLOTTING PARE ii Plot parent section(s) BLEN ii Plot normalized blended section(s) TRAN ii Plot transformed section(s) .DATA Plot transformed section data BLOW(b) Blowup airfoil plot region RESE(r) Reset current airfoil plot scale and origin REPL Replot current airfoil plot PARE, BLEN and TRAN are based on XFOIL plotting routines, with modifications and added code to support multiple section plots. These commands accept any integer arguments in the range of parents or loft stations, in any order. The argument 0 means 'all'. BLOW, RESE and REPL work similarly as in XFOIL. They work with the three XFOIL plots above. The plotting system is quite capable - for instance, one can use BLOW to zoom in to the leading edge of a multi-section BLEN plot, modify the parent section paneling with PANE, then execute REPL and the paneling of each section will update. Note that REPL will not function if the number of parent airfoils has changed (following PARE) or the number of loft stations (following BLEN or TRAN). Indices must be respecified. LOFTED SECTION DATA PLOTS The DATA command plots transformed section data under the following submenu, all plotted vs blade radius: Lofted Section Data Plots ------------------------------------------ 1 blade thickness 2 max thickness/chord 3 max thickness/chord x/c 4 max camber 5 max camber x/c 6 section area 7 leading edge radius 8 trailing edge thickness 9 trailing edge angle 10 zero-lift alpha - aero and esloft 11 beta - dfdc and esloft 12 chord ------------------------------------------ A toggle abscissa cm/inches B toggle local/global beta coordinates L limits for plot Z zoom plot with cursor R reset plot limits AN annotate plot H hardcopy plot W write plot data to file These should be self-explanatory. 10 and 11 indicate corrections made to beta data by ESLOFT. 3 and 5 are a good test of parent airfoil compatibility. While ESLOFT will create a smooth blade geometrically, radial aerodynamic "smoothness" is built into the parent airfoil set. Hence, ESLOFT does not remove the need for good parent airfoil design and selection. These data plots can assist the airfoil designer in refining the parent set. LOFT OUTPUT SAVB ii Save normalized blended section(s) to disk SAVT ii Save transformed section(s) to disk SAVR Save loft station radii to disk DISR Display loft station radii (in output units) As with section plotting, any sequence of points files can be saved to disk - normalized or transformed. The argument 0 will save all sections. SAVT writes transformed lofted points files to disk, dimensioned according to UNIT. Sections are scaled, rotated and translated according to blade geometry and loft settings, producing points files for lofting in 3D CAD software. Sections are rotated around the intersection of the pitch axis and camberline, then translated on the y (rotational) axis for specified circular arc dihedral. Blended (normalized) section output (SAVB) allows for exporting blended sections to XFOIL or other software for analysis and verification. The format is a named XFOIL .dat file. Sections will normally run in XFOIL with default ESLOFT paneling (nevertheless, repaneling in XFOIL is recommended for analysis in XFOIL). Transformed points files also will open in XFOIL, whether 2D or 3D. SAVR and DISR output station radius data, most essential when saving 2D points files. The radius data will be needed to set up loft stations in CAD software. Like transformed points files, station radius data are dimensioned according to UNIT. A station radius file is automatically written to disk whenever all transformed sections are written, whether 2D or 3D. PLOT CONTROLS HARD Hardcopy current plot .ANNO Annotate plot SIZE r Change absolute plot size Z Zoom U Unzoom These work essentially the same as in DFDC generally. Note that Zoom and Unzoom work with all plots and are independent of BLOW and RESE. BLOW adjusts the axes of airfoil plots while Zoom does not. *** Please report bugs *** END ! ESLOFT_doc